Twenty extremely low birthweight infants were treated with vancomycin (VCM). Their gestational age was 26.3 +/- 1.4 weeks (range 24.0-28.7 weeks) and their birthweight was 829 +/- 133 g (range 562-900 g). At the time of initial administration of VCM, postnatal age was 29.5 +/- 15.8 days (range 5-54 days). Vancomycin was administered in a dose between 9.3 and 11.0 mg/kg every 12 h for a period of 6.3 +/- 2.4 days (range 4-13 days). Serum and urinary specimens were obtained before and after (within 24 h following the last dose) VCM therapy. Serum creatinine and sodium, and urinary N-acetyl-beta-D-glucosaminidase (NAG) activity, creatinine, sodium, and beta-2-microglobulin (BMG) concentrations were measured. Fractional excretion of sodium (FENa) and NAG index (NAG:creatinine ratio) were calculated. There were no significant differences between the before and after VCM treatment in serum creatinine, urinary BMG concentration, FENa and NAG index. No infant showed any symptoms of renal insufficiency. However, the NAG index and FENa increased after treatment in 1 of 20 infants. Vancomycin is effective and safe in the treatment of serious infections in extremely low birthweight infants.