The results of both epidemiologic and experimental studies suggest that endogenous (and apparently exogenous) sex hormones and other reproductive variables have an important role in the development of human breast neoplasia. Nevertheless, no controlled study has ever addressed the possible effects of ovarian stimulation on the incidence and course of human breast cancer. Over the past decade the number of women undergoing follicular stimulation, especially during assisted reproductive technology procedures, has grown rapidly. Here we present 16 cases of young women who were treated by induction of ovulation, and subsequently were diagnosed with breast cancer. These women were drawn out of 950 cases of infertile women who underwent induction of ovulation at our fertility clinic over a 10-year period. The possible association between ovarian stimulation and promotion of breast cancer is discussed and the need for a controlled study is emphasized.