Twenty vancomycin pharmacokinetic studies were performed on 17 small infants who were receiving the antibiotic for treatment of documented infections. Fourteen patients were less than or equal to 41 weeks' postconception. In this group there was no statistical difference in mean elimination rate, volume of distribution, or clearance between neonates and infants 4 to 8 weeks of age. However, they had significantly lower clearance and prolonged mean beta-half-life than infants who were 3 to 6 months old (greater than 43 weeks' postconception). Vancomycin clearance was directly related to postconceptional age by linear regression analysis. beta-Half-life was influenced by the weight of the patient, volume of distribution, and gestational age. In view of the interpatient variability observed in the prematurely born infants, pharmacokinetic studies should be performed to determine the appropriate dose and intervals in vancomycin therapy.