OBJECTIVE: To investigate the effect of GH on myosin heavy chain (MHC) isoform composition, physical fitness and body composition in GH-deficient (GHD) patients. DESIGN: Twenty-two GHD patients were randomized in a double blind manner and half were treated with recombinant human GH (rhGH) and half were treated with placebo for 6 months. Twelve age-matched controls were also included in the study. METHODS: MHC isoform composition in biopsies obtained from the vastus lateralis muscle was determined using SDS-PAGE. Physical fitness was determined on a bicycle ergometer and body composition was determined using bioelectrical impedance analysis. RESULTS: More MHC IIX (28.9 +/- 4.1% and 10.0 +/- 3.1% in GHD and controls respectively (means +/- S.E.M.)) and less MHC I (36.2 +/- 2.4% and 51.7 +/- 3.9% in GHD and controls respectively (means +/- S.E.M.)) were present in the GHD patients compared with the controls. No significant difference in the amount of MHC IIA was detected. Linear regression was used to determine the relationship between variables. There were no significant relationships between the concentration of insulin-like growth factor-I (IGF-I) or the body composition and the MHC composition. Maximal oxygen uptake (VO(2)max) per kg body weight (BW) (litres/min per kg) correlated significantly with the amount of MHC I (r=0.60) and MHC IIX (r=-0.72) but not with the amount of MHC IIA (r=0.35). Treatment of GHD patients with rhGH for 6 months increased the concentration of IGF-I, lean body mass and decreased fat mass but had no effect on MHC composition and physical fitness. CONCLUSIONS: We conclude that a major part of the differences in MHC composition between GHD patients and age-matched controls can be explained by variation in physical fitness. The severity of the GHD and the body composition does not seem to be important for the MHC composition. Furthermore, treatment with GH for 6 months does not affect MHC composition in GHD patients.