Clinical experience suggests some individuals begin obesity treatment weighing more than they did at pretreatment assessment. Weight fluctuations between baseline screening and the first treatment session were examined among individuals enrolling in a group behavioral obesity treatment outcome study. Participants (N = 480, 94% female; 28% African American; M BMI = 35.7) were classified into those who started treatment having gained weight (≥+1.15% above screening weight), lost weight (≤−1.15% below screening weight) or remained weight stable. The majority of participants were weight-stable (61%) during the waiting period, but 23% lost weight (−2.36 ± 1.26 kg) and 16% gained weight (+2.11 ± 1.04 kg) between baseline screening and initiating treatment. Those who lost during the pretreatment period went on to have the greatest losses at 6-months (−8.9 ± 4.9 kg), with significantly greater weight losses than either the weight-stable (−6.1 ± 5.8 kg) or the weight-gain (−5.7 ± 5.8 kg) groups. Further, those who lost weight during the waiting period went on to attend a significantly higher proportion of treatment sessions and submitted more self-monitoring diaries than those who gained weight and those who stayed weight stable while waiting. Thus, pretreatment weight change was associated with treatment outcomes and may be relevant for research screening. Further, pretreatment weight change may be a clinical marker for likely success in behavioral weight control and as such warrants additional investigation to inform potential methods for enhancing outcomes for individuals in obesity treatment.