A ten-year follow-up study of 382 women with Stage I (T1N0M0) breast carcinoma revealed recurrence and/or death due to cancer in 16% of the patients. Among 134 patients (35%) with a primary tumor 1.0 cm or less in diameter (Group A), 7% had recurrences and 5% died of breast carcinoma. Recurrences were observed in 21% of the 248 women with a tumor 1.1-2.0 cm in diameter (Group B), and 15% died of disease. These differences in recurrence and mortality rates were statistically significant. All recurrences were due to infiltrating duct or lobular carcinoma which accounted for 91% of the 382 carcinomas. Most strongly linked to recurrence was the finding of tumor emboli in lymphatics of the breast. This was found in 23 Group B patients and ten of them (43%) died of disease. No recurrences were observed among the seven Group A patients with lymphatic emboli. Other features associated with a significantly increased risk of recurrence were poorly differentiated carcinoma, marked lymphoid reaction to tumor, and menarche before age 12 years or after age 14 years. No combination of variables proved to identify a subset of patients with an especially increased or low risk of recurrence. Stage I patients with lymphatic tumor emboli in the breast surrounding a carcinoma 1.1-2.0 cm in diameter have a sufficient risk for recurrence to warrant consideration of adjuvant systemic therapy. A very low risk of recurrence was observed for the following: any tumor 1.0 cm or smaller; and tubular, medullary or colloid carcinoma up to 2.0 cm.