This report describes the first pediatric case of chronic renal failure as a complication of infectious mononucleosis in a 3 1/2 year old girl. The clinical features were marked at onset by proteinuria and later by nephrotic syndrome. The evolution to chronic renal failure took about two years. In the renal biopsy, lesions characteristic of interstitial nephritis were observed, associated with focal and segmental glomeruloesclerosis and deposits of C3 in granular and discontinued form. Renal participation in infectious mononucleosis is not unusual and fundamentally it is manifested by proteinuria and abnormalities of urinary sediment, with interstitial nephritis as a characteristic lesion in the renal biopsy.