Background: In HER2-positive breast cancer, time elapsed between completion of (neo)adjuvant trastuzumab and diagnosis of metastatic disease ('trastuzumab-free interval', TFI) is crucial to choose the optimal first-line treatment. Nevertheless, there is no clear evidence to support its possible prognostic role., Methods: In the Adjuvant Lapatinib and/or Trastuzumab Treatment Optimisation (ALTTO) trial, patients with HER2-positive early breast cancer were randomised to 1 year of either trastuzumab alone, lapatinib alone, their sequence or their combination. This exploratory analysis included only patients in the trastuzumab alone or trastuzumab plus lapatinib arms who developed a distant disease-free survival (DDFS) event. Overall survival (OS) was defined as time between date of DDFS event and death; age at diagnosis, tumour size and hormone receptor status were the variables included in the multivariate models., Results: Out of 8381 patients included in ALTTO, 404 patients in the trastuzumab alone and trastuzumab plus lapatinib arms developed a DDFS event, of which 201 occurred < 12 months (group A) and 203 >12 months (group B) after completion of adjuvant trastuzumab. No significant difference in location of first DDFS event was observed (p=0.073); a numerically higher number of patients in group A than in group B developed brain metastasis (26% vs 15%). Choice of first-line therapy differed between the two groups (p=0.022): in group A, more patients received lapatinib (25% vs 11%) and less pertuzumab (8% vs 17%). Median OS was 29.3 and 18.4 months in groups B and A, respectively (adjusted HR 0.69; 95% CI 0.54-0.89; p=0.004). The longer OS for patients in group B was observed across the analysed subgroups without interaction according to hormone receptor status (p=0.814) nor type of administered adjuvant anti-HER2 treatment (p=0.233)., Conclusions: TFI has prognostic value in patients with HER2-positive early breast cancer treated with adjuvant trastuzumab-based therapy. TFI is a valid tool to better individualise clinical recommendations and to design future first-line treatment trials for metastatic patients., Competing Interests: Competing interests: ML acted as a consultant for Roche and Novartis, and received speaker honoraria from Theramex, Roche, Takeda, Novartis, Pfizer and Lilly outside the submitted work. NFP acted as a consultant for Lilly; received speaker honoraria from AstraZeneca, Novartis, Lilly and Roche-Genentech, travel support from Novartis and research grants from Daiichi Sankyo, MSD and BMS outside the submitted work. OW reports employment at Novartis. LDM acted as a consultant for Roche, Novartis, MSD, Pfizer, Ipsen, AstraZeneca, Genomic Health, Lilly, Seattle Genetics, Eisai, Pierre Fabre, Daiichi Sankyo; received speaker honoraria from Roche, Novartis, Lilly and MSD, and travel grants from Roche, Pfizer and Celgene outside the submitted work. RC received speaker fees from Boehringer-Ingelheim, AstraZeneca and Janssen, and travel support from AstraZeneca and Pfizer, outside the submitted work. AM-A received research grants from GSK/Novartis (to the institution) outside the submitted work. MJP served as board member of Oncolytics; received honoraria from AstraZeneca, Camel-IDS, Crescendo Biologics, Debiopharm, G1 Therapeutics, Roche-Genentech, Huya, Immunomedics, Lilly, Menarini, MSD, Novartis, Odone, Periphagen, Pfizer, Roche, Seattle Genetics, research grants from AstraZeneca, Lilly, MEDSD, Novartis, Pfizer, Radius, Roche-Genentech, Servier and Synthon (to the institution) outside the submitted work. EdA received honoraria and/or advisory board from Roche/GNE, Novartis, Seattle Genetics and Zodiac, travel grants from Roche/GNE and GSK/Novartis, research grants to his institution from Roche/GNE, Astra-Zeneca, GSK/Novartis and Servier outside the submitted work; his institution has received research grants for the conduct of ALTTO., (© Author (s) (or their employer(s)) 2020. Re-use permitted under CC BY-NC. No commercial re-use. Published by BMJ on behalf of the European Society for Medical Oncology.)