Multiple bilateral fibroadenomas are uncommon. This finding in four women who had received renal transplants prompted further inquiry. A prospective study was performed on 39 women under the age of 55 years who had received a renal transplant at least 1 year earlier. Clinical examination and breast ultrasonography were performed. Factors considered included immunosuppressive therapy, concurrent medication and renal function. Blood was taken for estimation of oestradiol, prolactin, follicle-stimulating hormone (FSH) and sex hormone binding globulin levels. Fibroadenomas were found in 13 of 29 women who had received cyclosporin A: multiple in ten and bilateral in five. No abnormal breast findings were seen in 10 patients immunosuppressed with steroids and azathioprine alone (chi 2 = 7.30, 1 d.f., P < 0.01). Serum oestradiol concentration was raised in women with fibroadenomas compared with that in those with normal breasts (P < 0.05) and the level of FSH was lower (P < 0.01). Cyclosporin A may act on breast fibroblasts by humoral mechanisms and direct action.