Glomerular filtration rate (GFR) was measured in two groups of cancer patients. In 20 patients, glomerular filtration rate was measured simultaneously with 51Cr-EDTA, 99mTc-DTPA and the X-ray contrast agent iohexol as markers, and with a complete set of eight blood samples during 24 hours. In a second group of 120 patients, we used 99mTc-DTPA only and tested various simplified methods, based on one or two blood samples. Glomerular filtration rate was also calculated from serum creatinine. There was excellent agreement between the values measured with the three markers, and, in the same group of patients, very good agreement with the results of simplified methods. The larger study carried out with 99mTc-DTPA on the second group of patients, confirmed a very good agreement between methods based on the slope of the plasma curve and a method based on one blood sample only. The correlation was worse between the values obtained by any of the radionuclide methods and those calculated from serum creatinine. The latter method should therefore not be used for determination of renal function.