Mammary carcinoma in women less than 30 years old appears to have a slightly poorer prognosis than in older women. Our study of 135 women in this age group suggests several explanations for this: 1. about 10% of the patients were pregnant or lactating—conditions associated with a higher incidence of axillary lymph node metastasis and lethal outcome; 2. incidence of axillary metastasis was slightly higher in younger women, regardless of pregnancy; 3. carcinoma frequently (17%) developed in the opposite breast, and 4. patients with 1 or 2 positive axillary lymph nodes did poorer than expected. Despite the overall unfavorable outlook, crude survival is improved by the relative infrequency of deaths from causes other than cancer, as contrasted with that of older women. Also, the prognosis for women under 30 is improved by the relatively high proportion (24%) of low-grade, infrequently metastasizing tumors, such as medullary, intraductal, juvenile, papillary, and well-differentiated carcinomas. When only infiltrating duct carcinoma was considered, 5-year survival in young women was 50% at 5 years and 37% at 10 years. Patients without axillary metastasis did as well as older women with breast cancer, since 81% were living at 5 years and 74% at 10 years.