ObjectiveThis study was designed to examine the variability in timing of peak weight loss in behavioral treatment (BT), that is, when treatment participants reach their greatest amount of cumulative weight loss.MethodsSecondary data analyses were conducted, drawing upon data from two clinical trials that provided 12 months of group‐based BT, with sessions meeting less frequently in the second half of the treatment year. Weight was measured at the beginning of each treatment session.ResultsCumulative proportions of participants reaching peak weight loss were as follows: 25.0% of study 1 and 20.0% of study 2 participants by month 4; 43.2% of study 1 and 52.2% of study 2 participants by month 6; 54.5% of study 1 and 77.8% of study 2 participants by month 8. Among participants who peaked after 4 months, the mean amount of additional weight loss achieved between 4 months and the peak session was 4.74 kg in study 1 and 4.07 kg in study 2.ConclusionsSubstantial variability in the timing of peak weight loss was observed, with larger than expected proportions of participants reaching their greatest amount of cumulative weight loss in the final months of treatment. This variability may create methodological and clinical challenges.