Increased dietary fat intake and rate of breast epithelial cell proliferation have each been associated with the development of breast cancer. The goal of this study was to measure the effect of a low fat, high carbohydrate diet on the rate of breast epithelial cell proliferation in women at high risk for breast cancer. Women were recruited from the intervention and control groups of a randomized low fat dietary intervention trial, breast epithelial cells were obtained by fine needle aspiration, and cell proliferation was assessed in these samples using immunofluorescent detection of Ki-67 and PCNA. The effects of needle size and study group on cell yield and cytologic features of the cells were also examined. Fifty three women (20 in the intervention group and 33 in the control group) underwent the biopsy procedure. Slides from 38 subjects were stained for Ki-67 and from 14 subjects for PCNA. No cell proliferation (fluorescence) was detected for either Ki-67 or PCNA in any of the slides. Epithelial cell yield and number of stromal fragments were greater with a larger needle size. Numbers of stromal fragments and bipolar naked nuclei were greater in the low fat as compared to the control group but no differences in epithelial cell yield were observed between the two groups. This study confirms that fine needle aspiration biopsy is a feasible method of obtaining epithelial cells from women without discrete breast masses, but suggests that cell proliferation cannot be assessed using Ki-67 and PCNA in such samples.