Background. Inhaled corticosteroids are currently the cornerstone of asthma treatment. Some studies of high-dose fluticasone propionate in patients with no or mild asthma have, however, suggested substantial systemic absorption. We investigated the pharmacokinetics of fluticasone propionate in patients with asthma receiving appropriate doses for severity. Methods. We did a double-blind, randomised, crossover study in 11 patients with asthma and 13 matched healthy controls (age 20-65 years; asthma patients forced expiratory volume in 1 s <75% and stable on high-dose inhaled corticosteroids). Patients received one 1000 μg intravenous dose or 1000 μg daily for 7 days inhaled (via spacer device) fluticasone propionate. In the 12 h after dosing, we monitored plasma fluticasone propionate and cortisol concentrations by mass spectrometry and competitive immunoassay with use of direct chemiluminescence. Analysis was by intention to treat. Findings. After inhalation, geometric mean values were significantly lower in the asthma group than in controls for fluticasone propionate plasma area under curve (1082 [95% CI 850-1451] vs 2815 pg mL-1 h-1 [2262-3949], -62% difference [45-72]; p <0.001), maximum concentrations (117 [91-159] vs 383 pg/mL [302-546], -68% [-50 to -81]; p <0.001), and systemic bioavailability (10.1 [7.9-14.0] vs 21.4% [15.4-32.2], -54% [-27 to -70]; p = 0.001). Intravenous-dose clearance, volume of distribution at steady state, plasma half-life, and mean residence time, were similar in the two groups. Less suppression of plasma cortisol concentrations was seen in the asthma group than in controls 4-12 h after inhaled fluticasone propionate. Interpretation. Systemic availability of fluticasone propionate is substantially less in patients with moderate to severe asthma than in healthy controls. Inhaled corticosteroids that are absorbed through the lungs need to be assessed in patients who are receiving doses appropriate for disease severity, and not in normal volunteers.