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Objective: To clarify whether the rate of decline in stimulated C-peptide (SCP) from 2 to 15 months after diagnosis has changed over an interval of 27 yr., Research Design and Methods: The rate of decline in SCP levels at 1, 2, 3, 6, 9, 12, and 15 months after diagnosis was compared in four paediatric cohorts from Scandinavian and European countries including 446 children with new onset type 1 diabetes (T1D, 1982-2004). Findings were evaluated against 78 children (2004-2009) from the TrialNet studies., Results: The mean rate of decline [%/month (±SEM)] in SCP for a 10-yr-old child was 7.7%/month (±1.5) in the 1982-1985 Cohort, 6.3%/month (±1.7) in the 1995-1998 Cohort, 7.8%/month (±0.7) in the 1999-2000 Cohort, and 10.7%/month (±0.9) in the latest 2004-2005 Cohort (p = 0.05). Including the TrialNet Cohort with a rate of decline in SCP of 10.0%/month (±0.9) the differences between the cohorts are still significant (p = 0.039). The rate of decline in SCP was negatively associated with age (p < 0.0001), insulin antibodies (IA) (p = 0.003), and glutamic acid decarboxylase-65 (GAD65A) (p = 0.03) initially with no statistically significant effect of body mass index (BMI) Z-score at 3 months. Also, at 3 months the time around partial remission, the effect of age on SCP was significantly greater in children ≤5 yr compared with older children (p ≤ 0.0001)., Conclusions: During the past 27 yr, initial C-peptide as well as the rate of C-peptide decline seem to have increased. The rate of decline was affected significantly by age, GAD65A, and IA, but not BMI Z-score or initial C-peptide., (© 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)