Data from animal experiments and human correlation studies strongly support the dietary fat-breast cancer hypothesis. Moreover, a causal relation between dietary fat and breast malignancy is biologically plausible. Negative findings from recent analytic epidemiologic studies of dietary fat and breast cancer, however, have fueled the notion that the hypothesis is no longer viable. We argue that only limited conclusions should be drawn from epidemiologic studies to date because of the narrow range of dietary fat intake among subjects and the substantial measurement error in dietary assessment. Although many doubts remain about the dietary fat--breast cancer hypothesis, the question is of such importance that intensive efforts at designing better studies of the hypothesis are urgently needed. Such studies might include (1) laboratory investigations in humans that examine possible mechanisms for the effects of fat, (2) large, prospective epidemiologic studies, and (3) randomized, controlled diet trials.