During a 4-year period 4684 nosocomial infections occurred in a university pediatric hospital which admitted 78,120 patients (nosocomial infection rate (NIR) = 6.0). NIR varied from 0.17 to 14.0 on different wards or services; the highest rates (greater than or equal to 5.6) were found in the Neonatal Intensive Care Unit, infant neurosurgery, hematology/oncology, neonatal surgery, cardiology/cardiovascular surgery, Pediatric Intensive Care Unit and infant/toddler medicine areas. Infections were most common in patients less than or equal to 23 months (NIR = 11.5), were less common in the 2- to 4-year age group (NIR = 3.6) and occurred least frequently in patients greater than or equal to 5 years (NIR = 2.6). The median day of onset of infections was 15.3 days. The proportional frequencies of infections were: 35% gastrointestinal; 21% bacteremia; 16% respiratory (10% upper, 6% lower); 7% postoperative wound; 6% urinary tract; 5% skin (32% of these skin infections were related to intravascular lines); 5% eye; 3% cerebrospinal fluid; and 2% other. A similar proportional frequency of 379 infections in patients hospitalized for more than 100 days was observed. The etiologic agents were Gram-positive bacteria (50%), viruses (23%), Gram-negative bacteria (18%), fungi (4%) and mixed/other (5%).