BACKGROUND: Prader-Willi syndrome (PWS) is associated with an inappropriate proportion of fat mass (FM) to non-FM compared to simple obesity. Altered body composition in PWS resembles that seen in subjects with GH deficiency, in which a reduction of lean body mass (LBM) is observed. The low LBM may contribute to the reduced motor skills seen in PWS patients. AIM: The objective of the study was to investigate the effects of GH therapy on exercise capacity and body composition in a group of adult subjects with PWS. SUBJECTS AND METHODS: Twelve PWS adults (7 males and 5 females, aged 26.4+/-4.4 yr, body mass index 44.3+/-4.6 kg/m2) participated in the study. Body composition analysis and exercise stress test were carried out throughout the 12 months GH therapy. Body composition was measured by Dual Energy X-ray Absorptiometry. Physical performance was evaluated using treadmill exercise test. Exercise intensity was expressed as metabolic equivalents (MET, 1 MET= 3.5 ml O2 kg(-1) min(-1)). Statistical analysis was performed by repeated-measures analysis of variance followed by post-hoc analysis with t test for paired data for comparisons among the different follow ups. RESULTS: Compared to baseline GH therapy increased LBM at 6 (p