Mesterolone, 150 mg daily by mouth, was given to 26 patients (10 men, 16 women) with renal anaemia on chronic haemodialysis (3 times for 5 hours). At the beginning of treatment the patients had been dialysed for at least 6 months under stable conditions: iron deficiency had been excluded or treated. Progressive improvement in the anaemia was observed during the treatment period. After 39 months the haemoglobin concentration had risen from 74 +/- 4 g/l to 95 +/- 5 g/l, haematocrit from 0.22 +/- 0.01 to 0.28 +/- 0.02, and the red-cell count from 2.44 +/- 0.12 X 10(12)/l to 3.09 +/- 0.2 X 10(12)/l. Side effects were rare; some patients developed increased appetite with a rise in body weight, while some women developed acne or hirsutism. There was no effect of mesterolone on liver function. The results indicate that mesterolone can favourably influence renal anaemia and that the side effects of this testosterone derivative are not such as to prohibit its use in women.