Thirty-four gross and histologic features of the primary tumor in 231 cases of clinical Stage I and II invasive breast cancer were reviewed in an attempt to identify features which might correlate with an increased risk of local recurrence within the breast following biopsy and primary radiation therapy. Local recurrence risk at 5 years was calculated for each feature studied. While results are reported in terms of 5-year actuarial local recurrence risk, not all patients were followed for 5 years (median follow-up period, 44 months). Patients with cases in which the biopsy was less than excisional had a considerably greater actuarial risk of local recurrence at 5 years than those in which the biopsy was excisional (36% versus 8%; P = 0.0005). Among 154 infiltrating ductal carcinomas treated by excisional biopsy prior to radiotherapy, histologic features associated with a significantly increased local recurrence risk at 5 years were the combination of extensive intraductal involvement by carcinoma and high nuclear grade and/or high mitotic index. Twenty-seven tumors demonstrated this constellation of features, and there was a 5-year actuarial local recurrence risk of 39% among this group. The local recurrence risk for the remainder of the population was only 4% (P less than 0.0001). Thus, through pathologic examination of the primary, the authors identified a subgroup of patients with a considerably increased risk of local recurrence following biopsy and primary radiotherapy.