Severe malnutrition is known to affect the pharmacokinetics of isoniazid (INH) in children. However, the effect of moderate malnutrition, which may be more prevalent, is not known. INH was administered to 20 children with tuberculosis at a single dose of 5 mg/kg, and serial blood samples were collected. The serum INH concentrations were higher in the undernourished group but the pharmacokinetic parameters were comparable with those in the normal nutrition group. Weight gain was significantly more in the undernourished group after 1 month of treatment. The study suggests that INH pharmacokinetics may not be significantly altered in children with moderate malnutrition.