Associations between breast cancer risk factors and histologic types of invasive breast carcinoma were studied in 2,728 patients. Lobular and tubular carcinomas occurred with increased relative frequency in most high-risk groups. The proportion of these types increased with age to a maximum at 45–49 years and decreased in the following decade. Significantly increased proportions of lobular and tubular carcinomas were also associated with high-risk countries, prior benign breast biopsy, bilateral breast cancer, concurrent mammary dysplasia, high age at first live birth, never-pregnant patients compared to those with a first live birth before age 20, private pay status, and length of education. Nonsignificant increases were associated with family history of breast cancer, less than 5 live births, less than 25 months total of breast feeding, use of oral contraceptives or IUD, and high occupational class. As a general trend, the higher the overall relative risk, the higher the proportion of lobular and tubular carcinomas. The occurrence of other histologic types also increased with increased breast cancer risk, but to a smaller degree than for lobular/tubular carcinomas. It is suggested that all hormonally related, socio-economic and geographic risk factors exert their effect by selectively increasing the number of lobular cells at risk. Family history of breast cancer and age over 49 years did not follow the general trend of parallel increases in the proportion of lobular/tubular carcinomas and breast cancer risk, and may operate through other mechanisms.