Numerous centers perform glomerular filtration rate (GFR) measurements on patients undergoing renal imaging with Tc-99m DTPA. GFR measurement, however, does involve multiple blood samples taken over a 4-hour period, and this has led to attempts to simplify the technique by reducing the number of blood samples required and hence diminish the time taken to perform the test and the inconvenience to the patient. Four different simplified techniques for measuring GFR that have been reported in the literature are compared with a reference 2 blood sample method. Three of the methods do not require any blood samples but have standard errors of greater than 20 ml/min in adults and greater than 14 ml/min in children. The other method requires one blood sample, and if this is taken at 2 hours postinjection, the standard error is 9 ml/min in adults and 5 ml/min in children. The latter method is suitable for routine use in renography when accuracy is not of paramount importance.