In 10 of 13 patients with unilateral testicular cancer and subsequent invasive cancer or carcinoma in situ in the remaining testis, the follicle stimulating hormone (FSH) level was elevated after the first orchiectomy and before further treatment. In only 4 of 26 comparable control patients was the FSH level raised. This may be because elevated serum FSH often reflects disturbances in spermatogenesis and fertility, the latter being a known risk factor for testicular cancer. An elevated FSH level that occurs after orchiectomy for unilateral testicular cancer and before further treatment identifies patients at high risk of developing a tumour in the remaining testis.