Background Bronchiolitis is a common respiratory disease in young children. Evidence suggests CysLTs are involved in its pathophysiology and treatment with montelukast (MNT) may be beneficial. Since there are no MNT pharmacokinetic (PK) data in children aged 3–6 months, the single-dose population estimate of AUC(0-∞) (area under the concentration-time curve[AUCpop]), maximum plasma concentration[Cmax], and time to Cmax[Tmax] of MNT 4-mg oral granules were investigated. Data were compared to historical data in children aged 6–24 months. Methods 14 children aged 3–6 months with active bronchiolitis or a history of bronchiolitis with “asthma-like” symptoms had plasma samples obtained and assayed for MNT after a single, oral 4-mg dose of MNT granules. Due to sparse sampling, a population PK approach with a nonlinear mixed-effect, 1-compartment model with first order absorption and elimination was used to fit the data. Ninety-five (95) % confidence intervals (CIs) for the AUC pop ratio (3–6 months/6–24 months) were provided. Results MNT 4-mg oral granules, in children aged 3–6 months, yielded systemic exposure (AUCpop) (3644.3±481.5 ng•hr/mL) similar to that in children aged 6–24 months (3226.3± 250.0 ng•hr/mL), with an AUCpop ratio of 1.13[95% CI (0.83–1.55)]. Similarly, no clinically meaningful differences were seen for other PK parameters. Conclusion Systemic exposure after administration of 4-mg MNT oral granules is similar in children aged 3–6 months and 6–24 months. Clinical Pharmacology & Therapeutics (2005) 77, P39–P39; doi: 10.1016/j.clpt.2004.12.042