Based on retrospective studies, nephrotoxicity may occur in as many as 35% of adult patients receiving vancomycin and an aminoglycoside. Limited data are available about the incidence of nephrotoxicity in pediatric patients, especially when drug therapy is closely monitored. We prospectively evaluated the potential of nephrotoxicity in 90 infants and children (61 less than 1 year and 29 greater than 1 year of age) receiving concomitant vancomycin and gentamicin for a duration of 3 to 38 (mean 9) days. Vancomycin and gentamicin doses ranged from 20 to 60 (mean 35) mg/kg/day and 2.5 to 14 (mean 6.5) mg/kg/day. Peak and trough serum concentration of vancomycin ranged from 10 to 55 and 2 to 18 micrograms/ml, respectively. Gentamicin peak and trough serum concentration ranged from 4 to 9 and 0.5 to 2.0 micrograms/ml, respectively. Serum creatinine concentration prior to, during and at the end of therapy averaged 0.42, 0.40, and 0.43 mg/dl (p greater than 0.1), respectively. Clinical status and urinalysis results showed no evidence of renal toxicity. These data suggest that nephrotoxicity is uncommon in pediatric patients receiving a combined therapy with vancomycin and gentamicin, particularly when serum concentrations of gentamicin are within therapeutic range.