The literature on the occurrence of left and of right breast cancer in the United States and in foreign countries is reviewed. In addition, several new series of cases from cancer registries and hospitals are reported. An analysis of the data shows that breast cancer in women occurs more often in the left breast than in the right breast. This difference is greater in foreign countries than in the United States. It is also shown that there is a smaller left/right ratio for breast cancer in women over the age of 70; that there are regional differences in the United States, by color; and that single women appear to have lower left/right ratios for breast cancer than women who are ever married. In American males, the occurrence of left breast cancer over that of right breast cancer shows a greater difference than in females. Trauma, poor milk secretion, and other factors are discussed in relation to the observed differences by side of breast.