Breast cancers from 476 elderly patients, 70 years and older, operated on since 1972, were analyzed for proliferative activity, hormone receptors, and DNA content. Tumor proliferative activity, expressed as 3H-thymidine labeling index (3H-TdR LI), had a median value of 3.4%, which progressively increased from 1972 to 1990. Estrogen and progesterone receptors were present respectively in 83% and 61% of the cases; the positivity for estrogen receptors slightly increased with time. Aneuploid clones were detected in 74% of the cases, and this incidence was relatively stable during the time of observation. 3H-TdR LI, hormone receptors, and ploidy were generally unrelated to the local-regional extension of the disease in these elderly patients, in agreement with observations on cancer from younger patients. However, the absence of hormone receptors and the presence of aneuploidy were markedly indicative of fast cell proliferation. As in younger patients, these biologic findings in elderly patients could be considered as a complement to clinico-pathologic features in a 'risk-factor profile system' for treatment planning.