A randomized dietary intervention trial across 4 years examined diet, weight, and obesity incidence (body mass index [BMI] greater than or equal to 30 kg/m[superscript 2]) differences between study groups. Participants were 1,510 breast cancer survivors with BMI greater than or equal to 25 kg/m[superscript 2] at entry. Dietary intake was assessed yearly by telephone; weight and height were measured at clinic visits. Intervention participants consumed more fruit, vegetables, and fiber and less energy from fat than control participants during follow-up cross-sectionally (p less than 0.0001) and longitudinally (p less than 0.0001); weight did not differ between study groups at any follow-up visit, and significant weight change difference was observed between groups only in the 1st year (p less than 0.0001). Diet and weight results remained unchanged after stratifying by age and BMI. No difference in obesity incidence was found during follow-up (p greater than 0.10) among overweight members of either study group. Without specific efforts to reduce total energy intake, dietary modification does not reduce obesity or result in long-term weight loss. (Contains 2 tables and 2 figures.)