Objective: To determine the effect of an educational and environmental intervention on diet, body mass index, and waist circumference of men in substance addiction treatment. Methods: One hundred three racially/ethnically diverse men in 6 urban substance addiction residential treatment facilities in Upstate New York participated in weekly nutrition and food classes and food environment changes to increase healthful food choices. The main outcomes of this controlled, quasi-experimental, pre-post evaluation study were diet, body mass index, and waist circumference. Results: Forty-three (42%) participants with complete data reported significantly greater intakes of fruits and vegetables, lower intakes of calories from sweets and desserts, and a reduction in waist circumference (P less than or equal to 0.05) following the intervention, compared to the control period. Conclusions and Implications: An educational and environmental intervention can be effective in promoting positive dietary behavior and reducing waist circumference among men in residential treatment. These results need to be confirmed in a larger, randomized trial. (Contains 2 tables.)