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The pharmacokinetics of oral ranitidine were studied in 9 patients (ages 9.9 to 19.6 years) with cystic fibrosis (CF). Patients were evaluated at steady-state conditions, and the mean maximum serum concentration (Cmax) was 845.7 ± 448.1 ng/mL. To adjust for the variable drug dosing used among study patients, both Cmax and area under the concentration curve (AUC) were standardized to dose (CmaxST) and AUCST, respectively) and were 217.9 ± 87.9 ng/mL and 1038.0 ± 242.2 ng/mL • h. The elimination half-life (t1/2) was 2.7 ± 1.4 hours, and the apparent steady-state volume of distribution (Vdss) was 4.6 ± 1.7L/kg. The plasma clearance was 1.022 ± 0.290 L/kg/h. The Vdss in this study was greater than that previously reported in children with peptic ulcer disease. Statistically significant relationships between pharmacokinetic parameters and measures of disease severity were not observed in the study population. The pharmacokinetics of ranitidine in children and adolescents with CF may differ from those in children and adolescents without CF.