Glomerular filtration rates (GFR) were measured with 51Cr-EDTA in 38 patients (aged 40-75 years) with homozygous sickle cell disease and compared with serum beta 2 microglobulin concentrations in 38 patients and with creatinine clearance in 21 patients. GFR estimated with 51Cr-EDTA was closely correlated with single serum creatinine measurements and the inverse of serum beta 2 microglobulin. Creatinine clearance was also found to be correlated, but values were, on average, 32% below those obtained by the 51Cr-EDTA method, and this difference was significant. It is concluded that measurements of beta 2 microglobulin, single serum creatinine, and creatinine clearance are valuable indicators of GFR in homozygous sickle cell disease. Measurement of beta 2 microglobulin was a useful and reliable method of estimating GFR from single plasma measurements and is therefore a useful means of screening the population.