The role of routine mammography was assessed in the early detection of asynchronous contralateral breast cancer (ACBC). The breast cancer patient populations of two cities, Nijmegen and Eindhoven, The Netherlands, which were subjected to a well-defined follow-up program, were compared. The program consisted of regular physical examination and annual mammography in Nijmegen and physical examination only in Eindhoven. From 1975 until 1987, 24 ACBC patients were detected within a group of 880 breast cancer patients in Nijmegen (3%) and, from 1971 until 1984, 14 ACBC patients within a group of 411 patients in Eindhoven (3%). In Nijmegen, eight of the 23 evaluable patients (35%) had a contralateral tumor with a histologic size smaller than 10 mm or an in situ carcinoma, compared with one of the 14 of the Eindhoven patients (7%), whereas 18 of the 24 (75%) versus eight of the 14 patients (57%) were node-negative. Thus annual mammography is very likely a contribution in the early detection of contralateral breast cancer as compared with follow-up by regular physical examination only.