objectives: follow-up of children exposed to oxacillin during hospitalization focusing on adverse reactions. methods: patients were selected from the pediatric wards of two hospitals in fortaleza (hospital universitário walter cantídio-huwc and hospital infantil albert sabin-hias) from the first oxacillin prescription with a prospective cohort study between october, 2000 and july, 2001 (huwc) and july/2001 and march, 2002 (hias). patients' follow-up was performed by daily visits to the wards and medical charts and prescription analysis. suspected oxacillininduced adverse reactions (oxar cases) were notified and classified according to causality and severity. related statistic tests were completed. results: of the 130 patients exposed to oxacillin, 27 had oxar (20.8%). fever was the most frequent reaction (50%) followed by rash (35.7%). the majority of reactions were considered probable, for oxacillin was the only medication involved and 92.6% of the cases had moderate severity with the need of therapeutic interventions caused by oxar. a significant relation between oxacillin exposure time and oxar was determined as well as hospitalization time and the appearance of adverse reactions. exposure time over 14 days to oxacillin was established as a risk factor for oxar (relative risk = 5.49). conclusions: careful administration of oxacillin in children is recommended with established treatment duration. empiric and prolonged use must be avoided.