This study focuses on a five-year profile (1990-1994) in the utilization of antiinfectives in a neonatal intensive care unit (NICU) in Rio de Janeiro, Brazil. Total antiinfectives consumption, diversity, and possible determinants were analyzed. A descriptive, longitudinal, and retrospective drug utilization study design was adopted. Document research, observation, and semi-structured interviews provided information on the neonatal intensive care provided, and drug supply and daily hospital records provided data on antiinfectives use, based on the Anatomical Therapeutic Chemical (ATC) classification and the infant defined daily dose (DDDi) as units of measurement. The numerical results suggest an increase in total consumption of antiinfectives and a decrease in their diversity. Analyzed in view of the institutional profile and rational drug use, these results indicate that changes in the type of care provided during the study period were key determinants of antiinfectives use.