Ovarian stimulation in the treatment of infertility is far from physiological because patients and their ovaries are exposed to high concentrations of gonadotropins. Many studies have focused on the two most common side-effects of ovarian stimulation--ie, hyperstimulation and multiple pregnancy. We describe 12 patients in whom granulosa-cell tumour was discovered after ovarian stimulation treatment with clomiphene citrate and/or gonadotropins. Although we cannot prove a causal link between the tumour and the medication, investigations in animals have shown a relation between gonadotropin exposition and the development of granulosa-cell tumours. The possible relation of ovarian stimulation and granulosa-cell tumours in human beings has not been published before. We postulate three explanations for this finding; first, the granulosa-cell tumour is present in the ovary, waiting for a hormonal trigger; second, increased follicle stimulating hormone concentrations are oncogenic to granulosa cell; and third, the onset of the granulosa-cell tumour during ovarian stimulation is coincidental. We recommend that ovarian stimulation is done only if there is a valid indication after proper assessment of the ovaries, and that women who have had ovarian stimulation are followed for longer than at present.