This study was undertaken to investigate the long-term survival and the probability of "cure" in a group of 644 patients treated by mastectomy for T1 breast carcinoma. After a median follow-up of 18.2 years, 23% were dead of recurrent breast carcinoma, 3% were alive with recurrent disease, and 74% had not experienced a recurrence. The probability of recurrence was directly related to the initial extent of the disease. Overall, 16% of recurrences and 25% of deaths due to disease occurred in the second decade of follow-up. The proportion of recurrences detected in the second decade was inversely related to the stage of the primary tumor at diagnosis. When stratified by tumor size, T1N0M0 patients with tumors 1.0 cm or less in diameter had a significantly better 20-year recurrence-free survival (86%) than did T1N0M0 patients with tumors 1.1 to 2.0 cm (69%). When observed and expected survival curves were compared by the method of Brinkley and Haybittle, it appeared that 80% of T1N0M0 patients with tumors 1 cm or less might be cured at 20 years, whereas for those in the 1.1- to 2-cm group, the proportion cured was indeterminate, but might be as high as 70%. A potentially cured group could not be identified among T1N1M0 patients, but an estimated 52% of these patients did not have a recurrence within the nearly 20-year follow-up period. These data are important when one considers the proper role of adjuvant therapy for stage I disease. Patients with tumors larger than 1 cm and those with axillary lymph node metastases may have an improved recurrence-free survival as a result of systemic adjuvant treatment, while women in the T1N0M0 group with an especially favorable recurrence-free survival, particularly those with tumors 1 cm in diameter or smaller, might be spared adjuvant therapy.