The relationship of breast size both to breast cancer risk and to the laterality of the tumor was studied among 261 women diagnosed with breast cancer and 291 control subjects who were enrolled in the United States' Breast Cancer Detection and Demonstration Project from 1973 to 1980. Standardized measures of breast area were obtained by applying planimetry to bilateral screening mammograms taken four years before breast cancer was diagnosed in case subjects. The left breast was larger in 53 percent of women with breast cancer and in 60 percent of women in the control group; the difference in breast area by laterality was significant only among controls (P = 0.01). To assess breast cancer risk, breast area was categorized by quartiles, with the lowest quartile being the referent group. Risk was increased minimally among women with the largest breast area (odds ratios = 0.9, 0.9, 1.2); however, the point estimates were not statistically significant and there was no evidence of a linear trend. Left-sided diseased was diagnosed in 51 percent of women in the case group. Although the mean area of the breast with the malignancy did not differ significantly from the opposite breast, cancer developed in the larger breast of 57 percent of women with left- and 46 percent of women with right-sided disease. Breast size was associated with cancer of the left breast but not the right. However, these size differences were small since the area of the larger breast was less than 10 percent greater than the smaller breast among half of the case subjects.(ABSTRACT TRUNCATED AT 250 WORDS)