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Objective: To assess cidofovir plasma concentration after intralesional airway administration for recurrent respiratory papillomatosis. Design: Prospective study. Setting: Tertiary care teaching hospital. Patients and Method: The study comprised 21 patients (10 children and 11 adults). Plasma samples were collected at 10 and 45 minutes (T10, T45) or at 10 and 60 minutes (T10, T60) after injection. The measurements of cidofovir were performed using a high-performance liquid chromatographic method. Results: Plasma samples were collected at T10 and T45 on 19 occasions from the children and on 17 from the adults. A linear relationship was found between plasma concentration and dose in children (mean dose 1.2 mg/kg; mean cidofovir plasma levels 0.91 and 0.81 μg/mL) but not in adults (mean dose 0.2 mg/kg; mean plasma levels 0.21 and 0.31 μg/mL). The same relationships were found between dose and area under the concentration/time curve (AUC). Four plasma samples were taken in children at T10 and T60: mean dose 1.2 mg/kg and mean plasma concentrations 1.11 and 1.24 μg/mL. Maximum plasma concentration averaged 34% (SD 11%) in children and 62% (SD 33%) in adults, with equivalent plasma level after intravenous infusion of the same dose. Conclusions: The cidofovir plasma levels were below those leading to toxicity. The levels and the AUC were dose dependent in children but not in adults. Diffusion from the injected site was greatest in a few adults and unpredictable. Because of the great individual variation in diffusion in adults, cidofovir should be used at less than the recommended intravenous dose to prevent any risk of systemic toxicity.