['The capacity to increase the tubular secretion of creatinine (TS(cr)) after an intravenous creatinine load (stimulated TS(cr)) has been found to be impaired in subclinical reduction of renal mass. We decided to investigate if this response was impaired in sickle cell anaemia (SCA) patients before there was evidence of deterioration of renal function.', 'Studies were done in 16 patients with homozygous SCA who had normal or supranormal glomerular filtration rate (GFR) and in 20 normal controls of similar median age (23 years). The tubular stress test (TST) consisted of 30-min clearance periods ([(125)I]iothalamate and creatinine) done before (baseline) and after (three successive post-stimulation periods) the intravenous infusion of 88.4 micromol (10 mg) of creatinine per kg of body weight.', 'Baseline studies showed that the SCA patients had higher GFR and lower serum creatinine concentration. After stimulation there were no changes in GFR. In contrast, creatinine clearance increased 2.3 times in normal but not in SCA patients (P<0.001) and the TS(cr) in the first post-stimulation period was 161+/-83 nmol/kg/min in SCA patients vs 286+/-93.2 in normal controls (P<0.001). The mean TS(cr) post-stimulation was also reduced in patients with SCA (123+/-52 nmol/kg/min vs 179+/-50 in normal controls, P<0.001). Since SCA patients had lower P(cr) values, separate analysis was made of post-load clearance periods in which P(cr) was comparable in patients and in normal controls (range 177-265 micromol/l or 2-3 mg/dl) and the reduction in TS(cr) was also present in SCA patients in these study periods.'] Is impaired creatinine secretion after an intravenous creatinine load an early characteristic of the nephropathy of sickle cell anaemia?