Because of the increasing frequency of breast conserving surgery, the significance of multifocal breast carcinoma has been reexamined. Multifocal breast carcinoma can be detected in at least 30% of radical mastectomy specimens, even if the index tumor is small and non-palpable. The actual percentage depends on the method of examination of the breast. Simulated tylectomy performed on mastectomy specimens has revealed potential foci of residual tumor in up to 80% of cases. However, the majority of these are close to the index tumor and are generally included within the usual margin of normal tissue removed along with the carcinoma. The difference in the recurrence rate following tylectomy with and without post-operative irradiation (approximately 25% vs 5%) indicates that these foci are clinically important. Most of the "recurrences" occur in the vicinity of the index tumor and within 4 years of the initial treatment. Beyond 5 years, "recurrences" tend to occur at sites remote from the index tumor and at a steady but much lower rate over the next decade; a rate that approximates to that of new primaries in the contralateral breast (1% per year). These late "recurrences" carry a relatively good prognosis. This biphasic pattern of recurrence emphasizes that multifocal breast cancer is a significant factor in both the short- and long-term management of patients treated by breast conserving surgery.