Patients with OSAS (obstructive sleep apnoea syndrome) demonstrate renal signs such as proteinuria, glomerular hypertrophy and focal glomerular sclerosis. We performed a clinical study to investigate the glomerular function in OSAS patients and the short-term effect of CPAP (continuous positive airway pressure) on it. OSAS patients underwent a sodium thiosulphate and p-aminohippurate double clearance test, polysomnography and ambulatory blood pressure monitoring before and a week after the induction of CPAP. Twenty-seven consecutive patients (24 males) with moderate-to-severe OSAS admitted to our hospital for the induction of CPAP, and 32 healthy donors for renal transplantation as controls participated in the study. Before treatment, the glomerular filtration rate, estimated by the sodium thiosulphate clearance test, was within normal range, and the renal plasma flow was significantly lower than normal in the OSAS patients, thus the FF (filtration fraction) value was much higher than normal. FF before CPAP was not significantly correlated with age, body mass index or blood pressure; however, indices of increased hypoxaemia correlated with increased FF values. Polysomnographic variables after CPAP showed significant improvements in all patients, and only the nocturnal blood pressures were slightly lower than before CPAP. In 21 patients who underwent the clearance test after CPAP, FF significantly decreased from 0.26 +/- 0.04 to 0.23 +/- 0.03 (P < 0.001). OSAS patients were generally in a glomerular-hyperfiltrating condition that appeared to cause the renal findings associated with OSAS. CPAP might prevent nephropathy by ameliorating the glomerular hyperfiltration in OSAS patients.