A pharmacokinetic investigation of administered corticosteroids was conducted in a 16-year-old girl with cystic fibrosis (CF). Equivalent doses of methylprednisolone were given orally and intravenously on consecutive days. Oral bioavailability was essentially 100 percent, ruling out impaired absorption. Drug half-life was shorter and both clearance and volume of distribution were greater in this patient than in normal young adults. From one patient it is impossible to conclude that this unusual disposition of corticosteroids is characteristic of CF. Given the interest in use of corticosteroids in this population, and the examples of other drugs with altered pharmacokinetics in patients with CF, further investigation is warranted.