The purpose of this study was to determine whether GH treatment of cystic fibrosis (CF) patients can result in an anabolic effect, i.e., increased weight gain, improved growth rate, nitrogen retention, and improved pulmonary function. Nine prepubertal endocrinologically normal CF patients (3 girls, 6 boys; chronological age (CA) 5.5-9.8 years, and bone age (BA) 4.5-9.0 years), received recombinant human growth hormone (rhGH) 0.3 mg/kg/week subcutaneously for a period of 12 months ( N = 8) or 9 months ( N = 1). Normal glucose tolerance was determined before treatment. Pulmonary function studies and anthropometric measurements were done every 3 months. Thyroid status, somatomedin C (SmC), BA, and routine chemistries were evaluated every 6 months. The pretreatment growth velocity averaged 5.7 +-0.3 (SE) cm/year and significantly increased to 7.8 5 0.4 (SE) cm/year during therapy, ( P < 0.01). Standard deviation scores (SDS) for height significantly increased during rhGH therapy as compared with pretreatment, ( P < 0.05). Weight of the patients during rhGH therapy did not significantly change during or after rhGH therapy. After therapy, all patients showed a significant increase in arm muscle area (AMA) and a significant decrement in arm fat area (AFA) ( P < 0.01). Net nitrogen anabolism was negative in all subjects before therapy but became more positive in five patients during rhGH therapy. Three patients acheived positive nitrogen retention. SmC values significantly increased from a mean value of 0.62 2 0.1 (SE) U/ml to 1.6 +-0.6 (SE) U/ml after therapy. BA advanced 1.0 ? 0.1 SE per year after treatment. Of the seven patients able to perform adequate pulmonary function testing, improvement occurred in FVC, FEV, o, and PEFR in 5, 5, and 4 patients, respectively, but these changes did not reach statistical significance. We conclude that biosynthetic rhGH therapy had a significant anabolic effect in CF patients as shown by increased growth velocity, SmC values, increased protein and decreased fat stores, and a positive or less negative net nitrogen retention in five of the patients.