We evaluated the growth rate and pubertal status of 54 adolescent and young adult men with cystic fibrosis and assessed the efficacy of short-term androgen therapy in promoting growth and pubertal development. Thirty-nine percent were below the fifth percentile in height and 8/28 (28%) between the ages of 14 and 18 had delays in pubertal development. Five male adolescents aged 13 years 8 months to 18 years 3 months were treated with testosterone and their conditions were prospectively evaluated. Growth rate increased from an entry mean of 2.2 cm/yr (range 0 to 4 cm/yr) to 7.2 cm/yr (3 to 10 cm/yr). We conclude that a brief course of testosterone appears to be a safe, effective means of improving growth rate in male adolescents with cystic fibrosis with delay of puberty.