Small urinary protein loss (low-grade albuminuria or microalbuminuria) may reflect altered permeability of the glomerular filtration barrier. In the present study, it was hypothesized that children with obstructive sleep apnea have an increased risk of microalbuminuria compared with control subjects without sleep-disordered breathing. Albumin-to-creatinine ratio was measured in morning spot urine specimens collected from consecutive children with or without snoring who were referred for polysomnography. Three groups were studied: (i) control subjects (no snoring, apnea–hypopnea index < 1 episode h−1; n = 31); (ii) mild obstructive sleep apnea (snoring, apnea–hypopnea index = 1–5 episodes h−1; n = 71); and (iii) moderate-to-severe obstructive sleep apnea (snoring, apnea–hypopnea index > 5 episodes∙h−1; n = 27). Indications for polysomnography in control subjects included nightmares, somnambulism and morning headaches. An albumin-to-creatinine ratio > median value in the control group (1.85 mg of albumin per g of creatinine) was defined as elevated. Logistic regression analysis revealed that children with moderate-to-severe obstructive sleep apnea, but not those with mild obstructive sleep apnea, had increased risk of elevated albumin-to-creatinine ratio relative to controls (reference) after adjustment for age, gender and presence of obesity: odds ratio 3.8 (95% confidence interval 1.1–12.6); P = 0.04 and 1.5 (0.6–3.7); P > 0.05, respectively. Oxygen desaturation of hemoglobin and respiratory arousal indices were significant predictors of albumin-to-creatinine ratio (r = 0.31, P = 0.01; and r = 0.43, P < 0.01, respectively). In conclusion, children with moderate-to-severe obstructive sleep apnea are at significantly higher risk of increased low-grade excretion of albumin in the morning urine as compared with control subjects without obstructive sleep apnea. These findings may reflect altered permeability of the glomerular filtration barrier related to nocturnal hypoxemia and sympathetic activation which are induced by obstructive sleep apnea.