Among the 911 cases of carcinoma of the breast treated between 1969 and 1978 there were 39 cases of bilateral primary cancer. Of these, 17 were synchronous tumours defined as cancers diagnosed and treated during the same primary hospital admission. The remaining 22 patients had tumours that were diagnosed separately (metachronous tumours), with a mean time interval of 5 years and 8 months between presentation of first and second primaries (range 6 months to 20 years). Of the metachronous tumours, 20 had presented within 7 years of the diagnosis of the first primary (2·2 per cent of all patients). The patients with synchronous disease fared badly, but patients in the metachronous group had a good prognosis. This is due, at least in part, to the inherent selection of patients in the metachronous group who have to survive the first primary tumour in order to develop the second. However, careful follow-up after the first primary may also account for the relatively good prognosis of this group. We therefore recommend continuing regular out-patient follow-up, with careful examination of the second breast, for all patients having undergone treatment for cancer of the breast.