The purpose of this study was to characterize the pharmacokinetics of vancomycin and to develop optimal dosage guidelines in infants. Thirteen infants between the ages of 13 to 183 days were enrolled. All had been born prematurely, and average gestational age, postconceptional age, and actual body weight were 29.8 weeks, 38.2 weeks, and 2.1 kg respectively. Multiple blood samples were obtained from each patient after 72 h of therapy. Serum inhibitory and bactericidal titres were determined for peak and trough samples. There were good correlations between total body clearance of vancomycin and both postconceptional age (r = 0.86) and actual body weight (r = 0.87). This information was used to develop vancomycin dosage guidelines in premature infants. The regression line for vancomycin daily dosage requirements vs postconceptional age may be useful for determining initial dosage recommendations. There were also good correlations between vancomycin serum concentrations and serum inhibitory and cidal titres. Peak and trough concentrations in the therapeutic range (peak, 25-35 micrograms/ml; trough, 5-10 micrograms/ml) corresponded to titres of greater than or equal to 1:8 and 1:2 to 1:8 respectively. Based on these data we suggest the following dosage guidelines for vancomycin: 10 mg/kg 12 hourly for 30-34 weeks postconceptional age and less than 1.2 kg actual body weight; 10 mg/kg 8 hourly for 30-42 weeks postconceptional age and greater than 1.2 kg actual body weight; 10 mg/kg 6 hourly for greater than 42 weeks postconceptional age and greater than 2.0 kg actual body weight. Thus, doses which are lower than currently recommended are needed for infants born prematurely.(ABSTRACT TRUNCATED AT 250 WORDS)