We found in an earlier study that participants in a short-term treatment program for obesity showed a good weight reduction (10.4 kg) 18 months after treatment terminated. The program included elements from cognitive therapy (CT) and psychoeducation. In the present study the efficacy of a slight modification of the same treatment program (cognitive treatment group) was compared with a behavioral program that included moderate-intensity physical activity and behavioral techniques (the control treatment group) in a randomized controlled trial. The primary effect variable was weight change 18 months after the end of therapy. Both treatment programs lasted for 10 weeks (2 hr/week), and thereafter the participants were weighed periodically over an 18-month period. The participants were obese women employed outside the home. Twelve of the participants did not receive treatment after randomization. Eleven of these participants had been randomized to the cognitive program, whereas the remaining participant was randomized to the control program. The mean age for those that began the 2 programs was 48.5 years, and the mean body mass index (BMI) was 36.6. For those who completed the treatment programs and participated in the 18-month follow-up, the baseline BMI was 34.7. One participant in the cognitive treatment group (n=16) and 6 in the control program (n=26) dropped out during treatment. Both per-protocol and intention-to-treat analyses were performed on the data. Fifteen participants (94%) completed the cognitive program. Of these, 13 (87%) participated at the 18-month follow-up. Their mean weight loss at treatment completion was 8.6 kg(SD=2.9) and 18 months later 5.9 kg (SD=5.4). Twenty participants (77%) completed the control program. Of these, 16 (80%) participated in the 18-month follow-up. Their mean weight loss at the end of treatment was 0.7 kg (SD=1.2), and 18 months later they showed an increase in weight of 0.3 kg (SD=4.3) as compared with baseline weight. The weight differences between the 2 program groups were highly significant (p<.01-.001) at all posttreatment weighings. In the intention-to-treat analysis, all participants who started the cognitive treatment (n=16) or control program (n=26) were included. The last observation carried forward was used for those who dropped out from therapy or from follow-up. Eighteen months after the end of therapy, the mean weight loss was 5.5 kg (SD=5.5) in the cognitive group, whereas the control group evidenced a weight loss of 0.6 kg (SD=5.5). The weight change differences between the 2 groups were highly significant at all follow-up weighings (p<.001). The low drop-out rate during the treatment period demonstrates that the participants found the 2 programs acceptable. The long-term efficacy of the cognitive treatment program seems to be satisfactory. With its group format and short treatment duration, the cognitive program is attractive from a cost-effective standpoint.