The pharmacokinetics of isoniazid (INH) was studied in children (0-196 months old) according to their acetylator phenotype, estimated from the metabolic acetyl INH/INH molar plasma concentration ratio (MR) measured 3 h after INH oral administration. There were 17 slow (MR < 0.48) and 17 fast acetylators (MR > or = 0.48). The mean apparent plasma clearance was significantly lower, the mean apparent volume of distribution higher and the half-life longer in the slow acetylator group (C1, 0.298 +/- 0.099 L/h/kg; Vd, 1.56 +/- 0.65 L/kg; t1/2, 3.88 +/- 01.89 h) than in the fast acetylator group (Cl, 0.528 +/- 0.234 L/h/kg; Vd, 1.06 +/- 0.45; t1/2, 1.64 +/- 1.1 h). The half-life decreased with age. An impaired isoniazid elimination was suggested in children less than three months old, which may be in favour of an individual dose adjustment in this population.