BACKGROUND: Historically, female breast carcinoma has been viewed as an etiologically homogeneous disease associated with rapidly increasing incidence rates until age 50 years, followed by a slower rate of increase among older women. More recent studies, however, have shown distinct age incidence patterns for female breast cancer when stratified by estrogen receptor (ER) expression and/or histopathologic subtypes, suggesting etiologic heterogeneity. MATERIALS AND METHODS: To determine if different age incidence patterns reflect etiologic heterogeneity (more than one breast cancer type within the general breast carcinoma), we applied "smoothed" age histograms at diagnosis (density plots) and a two-component statistical mixture model to all breast carcinoma cases (n = 270,124) in the Surveillance, Epidemiology, and End Results Program of the National Cancer Institute. These overall patterns were then reevaluated according to histopathologic type, race, and ER expression. RESULTS: A bimodal age distribution at diagnosis provided a better fit to the data than a single density for all breast carcinoma populations, except for medullary carcinoma. Medullary carcinomas showed a single age distribution at diagnosis irrespective of race and/or ER expression. CONCLUSIONS: Distinct age-specific incidence patterns reflected bimodal breast cancer populations for breast carcinoma overall as well as for histopathologic subtypes, race, and ER expression. The one exception was medullary carcinoma. Of note, medullary carcinomas are rare tumors, which are associated with germ-line mutations in the BRCA1 gene. These descriptive and model-based results support emerging molecular data, suggesting two main types of breast carcinoma in the overall breast cancer population.