The ability of several endocrine tests to distinguish between women with and without polycystic ovaries (PCO) has been studied prospectively in a series of 65 patients with functional oligo-amenorrhoea using ultrasonography as a reference test. In terms of overall diagnostic accuracy, the free androgen index (FAI) (94%) and the progestogen challenge (89%) gave similar results (Cl = -5 to 15%) but both were significantly better than all the other tests including measurement of serum LH (69%), total testosterone (71%) and androstenedione (74%) (minimum Cl = 6 to 23%). The FAI together with LH measurement was the most accurate of the combinations (97%) but this was not significantly better than the best of the combinations incorporating the progestogen challenge (92%; Cl = -3% to 13%). Given the limited availability of the assay for SHBG, these results show that assessment of oestrogen state is a useful method for categorizing patients with functional oligo-amenorrhoea.