Recent pharmacokinetic studies that included children found that serum drug levels were low compared to those of adults for whom the same dosages were used. This study aimed to characterize the pharmacokinetics of pyrazinamide and ethambutol in Malawian children and to examine the impact of age, nutritional status, and human immunodeficiency virus (HIV) infection. We conducted a pharmacokinetic study of children treated for tuberculosis with thrice-weekly pyrazinamide (n ؍ 27; mean age, 5.7 years) and of a separate group of children treated with thrice-weekly ethambutol (n ؍ 18; mean age, 5.5 years) as portions of tablets according to national guidelines. Malnutrition and HIV infection were common in both groups. Blood samples were taken just prior to oral administration of the first dose, and subsequent samples were taken at intervals of 2, 3, 4, 7, 24, and 48 h after drug administration. Serum drug levels were low in all children for both drugs; in almost all cases, the maximum concentration of the drug in serum (C max ) failed to reach the MIC for Mycobacterium tuberculosis. The C max of pyrazinamide was significantly lower in younger children (<5 years) than in older children. The C max of pyrazinamide was also lower for HIV-infected children and children with severe malnutrition, but these differences did not reach statistical significance. No differences were found for ethambutol in relation to age, HIV infection, or malnutrition, but the C max was <2 mg/liter in all cases. Studies of pharmacokinetic parameters and clinical outcomes obtained by using higher dosages of drugs for treatment of childhood tuberculosis are needed, and recommended dosages may need to be increased.There are very few pharmacokinetic (PK) studies of antituberculosis (anti-TB) drugs in children (8). Dosages for children are based on weight and extrapolated from data from studies with adults, yet pharmacokinetics for children, especially young children, is likely to be different than for adults. Studies of ethambutol and pyrazinamide have found lower plasma drug levels and shorter half-lives in children than in adults using the same dosages, and the authors have suggested that dosages per kilogram of body weight need to be higher for children than for adults (36,37). Similar conclusions were drawn from a recent study of isoniazid pharmacokinetics in South African children (29).Until recently, the use of the same dosage recommendation as for adults may not have been an important issue, since studies that followed these schedules for children found that outcomes were very good and serious adverse events were rare (1,2,6,33,34). These data suggest that adequate levels of drug were being achieved within a range that was safe. Recent reports of outcomes for child TB, however, have found much poorer treatment response than earlier studies (5,15,20,23).