Although animal experiments have consistently shown a positive relationship between breast cancer and energy intake, evidence from human studies remains inconclusive. In the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial cohort, 29,170 women, ages 55 to 75 years, who successfully completed a food frequency questionnaire (FFQ) at entry (1993-2001), were followed through 2007, and 1,319 incident breast cancers were ascertained (median time from FFQ completion to diagnosis, 4.4 years). Women in the highest quartile of energy intake, relative to the lowest, had modestly, but significantly, increased breast cancer risk [multivariate relative risk (RR), 1.21; 95% confidence interval (95% CI), 1.03-1.42; P(trend) = 0.03]. The inclusion of body mass index and physical activity in the model reduced risk slightly (RR, 1.18; 95% CI, 1.00-1.39; P(trend) = 0.07). However, in similar analyses using energy intake from a FFQ administered approximately five years after entry (27,428 women; 806 incident breast cancers; median time from FFQ completion to diagnosis, 2.7 years), women in the highest and lowest quartiles of energy intake had similar risk. When follow-up time after the first FFQ was divided into three 4-year periods, the multivariate RRs for high versus low energy intake increased from 1.21 to 1.37 to 1.55 with increasing time since dietary assessment. Although the divergent results for the two FFQs could be due to subtle questionnaire differences, our findings suggest a modest positive association between energy intake and postmenopausal breast cancer that strengthens with time since dietary assessment.