Patients with AIDS frequently develop circulating immune complexes (CIC) and elevated serum immunoglobulins which might predispose to renal disease. The latter has been recently described in >10% of adults with AIDS, but no such association has been reported in children. We studied 15 patients ([xmacr ] age 11 months, range 2-24) with proven AIDS (defined by an inverted T4/T8 ratio plus Kaposi's Sarcoma or unusual infections). All evidenced intermittent proteinuria (up to 55 mg/kg/day) and 7 had hematuria (up to 50 RBC's/HPF) and/or casts. Serum creatinine was normal ([xmacr ] 0.6 mg/dl). Elevated ESR (max. 116 mm/hr) was noted in all patients, increased CIC (ClQ and Raji) in 6 and high serum IgG (max. 4,910 mg/dl) in 10. Serum C3 and C4 were normal in all. Renal autopsy material was available in 7 patients of which 3 had abnormal findings: 1 with markedly increased mesangial matrix and nuclei, 1 with profuse electron-dense deposits in the mesangium, and 1 with immunofluorescent deposits of IgM and C3 in the mesangium. We conclude that AIDS is often associated with urinary and renal histological changes. These changes are not as severe as those in adults possibly due to the shorter duration of the disease, lack of associated complicating factors (drug addiction, etc.,) or to the renal and immune characteristics of children ; still they may carry prognostic significance.