Objectives: To evaluate white matter and grey matter T1-weighted (w)/T2w ratio (T1w/T2w ratio) in healthy controls and patients with multiple sclerosis, and its association with clinical disability., Methods: In this cross-sectional study, 270 healthy controls and 434 patients with multiple sclerosis were retrospectively selected from 7 European sites. T1w/T2w ratio was obtained from brain T2w and T1w scans after intensity calibration using eyes and temporal muscle., Results: In healthy controls, T1w/T2w ratio increased until 50-60 years both in white and grey matter. Compared with healthy controls, T1w/T2w ratio was significantly lower in white matter lesions of all multiple sclerosis phenotypes, and in normal-appearing white matter and cortex of patients with relapsing-remitting and secondary progressive multiple sclerosis (p≤0.026), but it was significantly higher in the striatum and pallidum of patients with relapsing-remitting, secondary progressive and primary progressive multiple sclerosis (p≤0.042). In relapse-onset multiple sclerosis, T1w/T2w ratio was significantly lower in white matter lesions and normal-appearing white matter already at Expanded Disability Status Scale (EDSS) <3.0 and in the cortex only for EDSS ≥3.0 (p≤0.023). Conversely, T1w/T2w ratio was significantly higher in the striatum and pallidum for EDSS ≥4.0 (p≤0.005). In primary progressive multiple sclerosis, striatum and pallidum showed significantly higher T1w/T2w ratio beyond EDSS=6.0 (p≤0.001). In multiple sclerosis, longer disease duration, higher EDSS, higher brain lesional volume and lower normalised brain volume were associated with lower lesional and cortical T1w/T2w ratio and a higher T1w/T2w ratio in the striatum and pallidum (β from -1.168 to 0.286, p≤0.040)., Conclusions: T1w/T2w ratio may represent a clinically relevant marker sensitive to demyelination, neurodegeneration and iron accumulation occurring at the different multiple sclerosis phases., Competing Interests: Competing interests: M. Margoni reports grants and personal fees from Sanofi Genzyme, Merck Serono, Novartis and Almiral. She was awarded a MAGNIMS-ECTRIMS fellowship in 2020. E. Pagani received speakers’ honoraria from Biogen Idec. A. Meani received speakers’ honoraria from Biogen Idec. L. Storelli received speakers’ honoraria from Biogen Idec. She is supported by a senior research fellowship FISM—Fondazione Italiana Sclerosi Multipla—cod. 2019/BR/009 and financed or co-financed with the ‘5 per mille’ public funding. S. Mesaros has received travel funding from Merck, Bayer Schering, Medis and Genzyme Sanofi; has received speakers’ honoraria from Merck, Novartis, Medis, Sanofy Genzyme, Roche, Hemofarm. J. Drulovic received speaker’s honoraria from Merck Serono, Bayer, Teva, Genzyme Sanofi, Medis, Roche, Hemofarm and Novartis; and has also received research grant support from the Ministry of Education, Science, and Technological Development of the Republic of Serbia (project 200110). F. Barkhof acts as a consultant to Biogen-Idec, Janssen, Bayer, Merck, Roche, Novartis, IXICO, and Combinostics; he has received sponsorship from EU-H2020, Dutch MS research foundation, NIHR, EU-IMI, Biogen and GE Heatlthcare. H. Vrenken has received research grants from Merck Serono, Novartis and Teva, speaker honoraria from Novartis and consulting fees from Merck Serono; all funds paid directly to his institution. A. Gallo received speaker and consulting fees from Biogen, Sanofi-Genzyme, Merck Serono and Teva. A. Bisecco received speaker’s honoraria and/or compensation for consulting service and/or speaking activities from Biogen, Roche, Merck, Celgene, Coloplast and Genzyme. D. Pareto has received speaking honoraria from Novartis and Sanofi-Genzyme and a research contract from Biogen Idec. J. Sastre-Garriga declares grants and personal fees from Genzyme, Biogen, Celgene, Merck, Bayer, Biopass, Novartis and Roche outside the submitted work; Dr Sastre-Garriga is Associate Editor of Multiple Sclerosis Journal and Scientific Director of Revista de Neurologia, outside the submitted work. O. Ciccarelli receives grant support from the UK MS Society, National MS Society, NIHR, EU-H2020, Spinal Cord Research Foundation, Rosetrees Trust, Progressive MS Alliance, Bioclinica and GE Neuro. She received speaker’ honoraria from Biogen and Merck-Serono and is a consultant for Novartis. She is Deputy Editor of Neurology, for which she receives an honorarium. M. Yiannakas has nothing to declare. J. Palace has received support for scientific meetings and honorariums for advisory work from Merck Serono, Novartis, Chugai, Alexion, Roche, Medimmune, Argenx, UCB, Mitsubishi, Amplo, Janssen. Grants from Alexion, Roche, Medimmune, Amplo Biotechnology. Patent ref P37347WO and licence agreement Numares multimarker MS diagnostics Shares in AstraZenica. Acknowledges Partial funding by Highly specialised services NHS England. P. Preziosa received speakers’ honoraria from Biogen Idec, Novartis, Bristol, Myers Squibb, Genzyme, Roche and Excemed. M.A. Rocca received speakers’ honoraria from Bayer, Biogen, Bristol Myers Squibb, Celgene, Genzyme, Merck Serono, Novartis, Roche and Teva, and receives research support from the MS Society of Canada and Fondazione Italiana Sclerosi Multipla. M. Filippi is Editor-in-Chief of the Journal of Neurology and Associate Editor of Radiology, Human Brain Mapping and Neurological Sciences; received compensation for consulting services and/or speaking activities from Almiral, Alexion, Bayer, Biogen, Celgene, Eli Lilly, Genzyme, Merck-Serono, Novartis, Roche, Sanofi, Takeda and Teva Pharmaceutical Industries; and receives research support from Biogen Idec, Merck-Serono, Novartis, Roche, Sanofi, Almiral, Eli Lilly, Teva Pharmaceutical Industries, Italian Ministry of Health, Fondazione Italiana Sclerosi Multipla and ARiSLA (Fondazione Italiana di Ricerca per la SLA)., (© Author(s) (or their employer(s)) 2022. No commercial re-use. See rights and permissions. Published by BMJ.)