Fifteen children followed as outpatients with chronic inflammatory disease of the colon were given sulfasalazine in doses from 1 to 4 gm/day (22 to 68 mg/kg, or 0.69 to 2.33 gm/m2). No correlation was found between the dose/m2 administered and the total serum sulfapyridine levels. However, 11 of 15 patients achieved SP levels ≥17 μg/ml, a level approximating that reputedly associated with therapeutic efficacy. Patients who were either slow acetylators or slow hydroxylators of sulfapyridine had total SP levels significantly higher than patients who were both rapid acetylators and hydroxylators (20.0±1.2 vs 14.6±1.6). Total SP serum levels were not correlated with the activity of the disease. No toxic levels (>50 μg/ml of SP) were encountered. We conclude that a dose of SASP in the range of 1.5 to 2.0 gm/m2 can be safely administered to children and is usually associated with serum SP levels considered in the therapeutic range. Although one-third of children are both rapid acetylators and hydroxylators and will have somewhat lower SP levels, the routine monitoring of SASP therapy with SP levels is not necessary for management of disease.