• Vancomycin hydrochloride levels were studied in 44 children (age range, 10 days to 10 years). These children received doses of vancomycin within current recommendations. Major variations in vancomycin levels were demonstrated in similar age groups and dosage regimens. For patients in the first month of life, second month of life, and older ages, respectively, 70%, 39%, and 30% of the peak determinations and 53%, 50%, and 23% of the trough determinations were greater than the desired level. Potentially toxic levels were found in eight patients.