The investigation is described of a 62-year-old female patient who presented with severe virilisation; her plasma testosterone concentration was grossly elevated at 37.9 nmol/L. Measurement of plasma androstenedione, dehydroepiandrosterone (DHEA) and DHEA-sulphate, urinary 17-oxosteroids and urinary 'free' cortisol suggested an ovarian source of androgens. An ultrasound scan indicated the presence of an ovarian mass which was removed and classified as a Sertoli-Leydig cell tumour (arrhenoblastoma) combined with a mucinous cystadenoma. Following operation plasma testosterone levels returned to normal. The excessive pre-operative testosterone production appeared to have had little effect on the plasma level of SHBG, since levels remained similar before and after removal of the tumour. Serum LH and FSH levels were higher post-operatively.