The relative accuracy of five simplified methods of measuring glomerular filtration rate was prospectively assessed using 99mTc-DTPA. The slope of the biological clearance curve, the 3-h volume of dilution and three renal tracer uptake methods (Piepsz, Gates and Nielsen) were concurrently compared with a multiple blood sampling reference method. The volume of dilution method from a single blood sample was markedly superior to the other four simplified methods which all had a similar degree of accuracy.