Angiotensin-1 converting enzyme inhibitors (ACEI) have been shown to reduce proteinuria in azotaemic diabetics and in other glomerulopathies, and such treatment has also slowed the development of experimentally-induced glomerulosclerosis in animals. We have treated 13 patients with focal segmental glomerulosclerosis (FSGS) and IgA nephropathy (IgAN) with Captopril 12.5 mg twice daily for six months and assessed their response in terms of 24 hour urinary protein excretion, blood pressure, glomerular filtration rate, effective renal plasma flow and derived values for filtration fraction and renal vascular resistance. A mean fall of 29 per cent in urinary protein excretion was observed over the six months treatment schedule. No significant changes were observed in other parameters of renal haemodynamics measured. We conclude that Captopril therapy in patients with FSGS and IgAN reduces urinary protein excretion consistently over a six month period, and that this may in the longer term retard the progression of their renal failure.