Objectives: Malnutrition is a negative prognostic indicator in patients with cystic fibrosis (CF) and may accentuate pulmonary decline. We tested whether megestrol acetate would have beneficial effects on growth in patients with CF and pancreatic insufficiency. Study design: We performed a randomized, double-blind, placebo-controlled study. All patients were taking replacement enzymes to compensate for pancreatic insufficiency. Patients (n = 17) were randomly assigned to receive either megestrol acetate or placebo. Results: The treatment group had a significant increase in weight-for-age scores compared with placebo and reached 100% of their ideal body weight within 3 months of initiating therapy. Weight gain included both fat and fat-free mass. Improved pulmonary function (forced vital capacity and forced expiratory volume in 1 second) was noted in the treatment group compared with placebo (P < .04). Reversible adrenal suppression was observed in the majority of patients who received megestrol acetate. Conclusions: Short-term use of megestrol acetate results in significant weight gain and improved pulmonary function in malnourished subjects with CF. Our study provides a controlled basis for this intervention, identifies important side effects, and provides the foundation for multiyear, longitudinal trials in a larger number of patients with CF.