Poor longitudinal growth and low body weight affect many persons with cystic fibrosis (CF). The Cystic Fibrosis Foundation reports that 28% of all persons with CF are below the 10th percentile for height and that 34% are below the 10th percentile for weight. Intensive nutritional supplementation has not resulted in sustained improvement in the poor linear growth and low weight in CF. Because of the significant impact of nutrition in CF, the anabolic effects of growth hormone (GH) may make the agent useful as adjunctive treatment for malnutrition and poor linear growth. To date, 24 patients with CF (16 boys; 87% Tanner stage 1) have been enrolled in the National Cooperative Growth Study. The average age at enrollment was 10.3 years, and there was significant delay in height in all patients (mean height age, 7.1 years). Bone age was also significantly delayed (mean delay, 3.0 years). The mean maximum stimulated GH level was 12.3 micrograms/L and the mean GH dose given was 0.291 +/- 0.038 mg/kg per week. After 1 and 2 years of treatment with GH the growth rate increased in all patients with available growth rate data. The growth rates in these children were slightly lower than in children who were treated with GH for idiopathic GH deficiency. The weight-for-height standard deviation scores improved significantly after 2 years of GH treatment. There were adverse reactions (glucose intolerance) to GH in only two patients; treatment was suspended in one of these patients but was continued in the other. National Cooperative Growth Study data indicate that treatment with GH increases linear growth and weight in prepubertal patients with CF. These data suggest that GH may be useful for treating malnutrition in CF.