Since 1982, a pediatric intensive care unit for neonates and pediatric patients up to 15 years of age has prospectively recorded every instance of use of antimicrobials, with the reasons for use, clinical and bacteriological parameters, and outcome. This approach encourages strict adherence to established protocols and provides a basis for discussing the rationale of each antimicrobial course. Effectiveness of protocols is evaluated annually and modifications or additions are introduced, as appropriate. Patterns in the proportion and nature of antimicrobials used to treat hospital-acquired infections can be monitored on the basis of the data collected. Changes in the nature of antimicrobials used, which may have repercussions on pathogen resistance to antimicrobials, are also monitored. Reasons for use of antimicrobials are categorized as follows: A = primary infection: B = secondary infection acquired in the ICU; C: secondary infection acquired in another unit or in another hospital; D = prophylaxis. In 1987, 46% of neonates and 59% of patients above one month of age were given antimicrobial agents; these figures are similar to those recorded during the previous years. Reasons for antimicrobial therapy were as follows in neonates: A = 48.5%; B = 40%; C = 1.2%; D = 10.3%; in patients above one month of age corresponding figures were: A = 23%; B = 44%; C = 0.9%; D = 31.9%. Among the neonates, the ampicillin-aminoglycoside combinations accounted for 41.5% of treatments (1/4th of these treatments were continued); in the older patients, penicillin G and ampicillin were the most commonly used antibiotics. In all age groups, hospital-acquired infections were mainly respiratory tract infections (approximately 50% in neonates and 80% in patients above one month of age). Staphylococcus aureus was the most prevalent organism; Pseudomonas was seen virtually only among the patients above one month of age with very prolonged endotracheal intubation. From 1983 through 1987, use of third-generation cephalosporins increased from 4.5% to 28.3% in neonates and from 5.5% to 9% for patients above one month of age. The changes identified over time should be interpreted in the light of changing patterns of disease; in particular, hospital-acquired infections among neonates increased twofold, probably as a result of the rising number of very-low-birth-weight infants.