Surgical implantation of breast prostheses for cosmetic purposes has become increasingly popular, and by 1981, it was estimated that three-quarters of a million women had had such an operation. The long-term potential risks, particularly of breast cancer, of such procedures have not been properly investigated. To evaluate the potential breast cancer risk, we have conducted a retrospective cohort study of 3111 women followed through various public and medical records for a total of 18,476 person-years, with a median of 6.2 years per person. The cases of breast cancer were detected by means of a computerized match with the Los Angeles County Cancer Surveillance Program, a population-based cancer registry. Overall, 15.7 breast cancer cases were expected and 9 were observed, a nonsignificant deficit [standardized incidence ratio (SIR) = 57 percent, 95 percent confidence limits: 26 percent, 109 percent]. The cancers were generally diagnosed at an early stage. Among the 573 women aged 40 or older at implantation, 7.1 cases were expected and 8 were observed (SIR = 113 percent). In women whose implants were performed before the age of 40, only 1 case was observed whereas 8.6 cases were expected (SIR = 12 percent, 95 percent confidence limits: 0.3 percent, 65 percent), a significant difference. These data do not support an increased risk of breast cancer following augmentation mammaplasty. The low breast cancer rate in women having augmentation mammaplasty at a young age that many such women may have a reduced amount of breast tissue, but data on this are unavailable.