Renal abnormalities in patients with obstructive sleep apnea syndrome (OSAS) have not been previously described. Medical records of patients who had been evaluated for possible sleep apnea syndrome and had had complete polysomnograms and urinalyses were reviewed to determine the frequency of proteinuria. High-grade proteinuria (greater than or equal to 3+ on urinalysis) was found in 6 of the 34 patients with obstructive sleep apnea, but in none of 34 patients in a control group matched for sex, age, and weight. In three patients, proteinuria was in the nephrotic range (3.5 g/24 h). The weight (mean +/- SD) of the patients with obstructive sleep apnea (112.7 +/- 35.3 kg) was not significantly different from the control group (109.2 +/- 30.3 kg). Microscopic examination of renal tissue in one patient with OSAS showed minimal changes. In four patients who were followed for 3 years, proteinuria improved after therapy for sleep apnea syndrome. We suggest that proteinuria may not be uncommon in patients with severe obstructive sleep apnea syndrome and may be reversible with correction of the sleep apnea syndrome.