In order to study the effect of renal disease on urinary estriol clearance rate, we have measured the concentrations of plasma and urinary estriol (E3) in pregnancies of 69 diabetic patients, 25 of whom had nephropathy. No correlation was found between endogenous creatinine clearance (CCr) and estriol clearance (CE3) rates (r = 0.07), and mean CE3 of diabetic patients with diminished CCr (less than 100 ml/min) was not significantly different from that of patients with normal CCr (greater than or equal to 100 ml/min). The ratios between total, unconjugated estriol and urinary estriol concentrations in patients with diminished CCr were not different from those patients with normal CCr. Cases where high plasma estriol and low urinary estriol concentrations coexisted were not found. It is concluded that in this group of diabetic patients, diminished CCr had no discernible effect on urinary CE3 possibly because renal tubular function remained intact. In patients with diabetic nephropathy either urinary or plasma E3 could be used to assess fetoplacental function.