The value of routine follow up of women treated for early breast cancer by mastectomy with or without postoperative radiotherapy was assessed retrospectively. Over eight years 546 patients made 6863 clinic visits, during which 192 first relapses were detected. Ninety three relapses were detected at scheduled (routine) visits and 99 at unscheduled (interval) visits. First relapses within the treated area or in the contralateral breast were detected significantly more commonly at routine visits than were first metastatic relapses (66/89 (74%) compared with 27/103 (26%)). Patients whose local relapse was detected at a routine visit had a significantly better survival than those whose local relapse was detected at an interval visit. A relapse that was potentially curable (local or in the contralateral breast) was detected at 66 (1%) of 6764 routine visits, but only 26 (39%) of these patients remained free of disease. It is concluded that the intensity of follow up of such patients could be reduced without any adverse effect on prognosis but with appreciable financial and other benefits.