A study was carried out in four patients with nephrotic diabetic nephropathy in order to determine if decreased creatinine clearance observed during prostaglandin E1 therapy was reversible on discontinuation of therapy. The patients received 40 micrograms prostaglandin E1 intravenously twice daily for 4 weeks and creatinine clearance and daily excretion of urinary protein were measured immediately before, during and 2 weeks after therapy. Total serum protein and serum albumin were also determined. There was a significant decrease in creatinine clearance during therapy and after therapy clearance increased but not significantly. It is concluded that decreased creatinine clearance during prostaglandin E1 therapy has a partial reversibility on discontinuation of the treatment.