Seventy-eight confirmed cases of second primary breast cancer in the contralateral breast were encountered over a 22-year period in 1332 women with invasive breast cancer treated in our department. Tumors were grouped into those simultaneously detected in both breasts or within 6 months of each other (synchronous, 1.6%) and those detected within more than 6 months (metachronous, 4.2%). The mean interval between metachronous cancers was 117 months. Patients with bilateral tumors were more likely to have a family history of breast cancer than those with unilateral disease. Women with metachronous tumors tended to be younger when diagnosed with the first carcinoma as compared with those having unilateral or synchronous bilateral cancers. No differences were noticed in size and lymph node status between the first or second tumor of bilateral cases in comparison to patients with unilateral disease. Significantly more (P < 0.05) first metachronous tumors were found to be lobular invasive cancers. Histopathologic type of the first tumor was the same as the second in 62.8 per cent of all cases. Concordance of estrogen receptor status between bilateral tumors was 71.4 per cent. Our results indicate that the risk of developing, a contralateral breast cancer is related to the patient's age, family history of breast cancer, and lobular histology of the tumor.