Background: Patients with Prader-Willi syndrome (PWS) due to maternal uniparental disomy of the chromosome 15 (UPD15) have fewer facial features, less hypopigmentation and higher levels of psychosis compared to subjects with deletion in chromosome 15 (del15q11-q13). PWS individuals carrying the larger type I (TI) deletion suffer from greater behavioral problems than patients with the smaller type II (TII) deletion. Few data are currently available on the relationship existing between endocrine abnormalities in PWS subjects and the different genotypes. Aim: To investigate the stimulated GH levels in PWS patients with different types of deletion and those with UPD15. Subjects and methods: Thirty-seven patients, 14 males, aged 17.5-41.2 yr, with PWS due to TI deletion (no.=6), TII deletion (no.=15) or UPD15 (no.=16), were studied. Pituitary GH secretion was evaluated by dynamic testing with GHRH+arginine. Results: Both the mean peak GH response and the integrated GH secretion (GH area under the curve and GH area under the curve corrected for basal values) for the UPD15 patients (4.6±1.6 μg/l, 241.6±71.7 μg/l/h and 228.3±71.6 μg/l/h, respectively) were lower than that observed in all subjects with del15q11-q13 (9.1±1.8 μg/l, 547.0±132.3 μg/l/h and 514.9±127.6 μg/l/h: p<0.005), as well as in TI (7.7±1.2 μg/l: p<0.02; 424.2±88.8 and 393.4±88.8 μg/l/h: p<0.05) and TII (9.6±2.6 μg/l, 587.9±174.2 μg/l/h and 555.4±167.6 μg/l/h: p<0.01) deletion groups. TI and TII groups had similar stimulated GH levels and integrated GH secretion. Conclusions: Our results point at differentiating the pattern of GH secretion by genetic subtypes, with higher GH responses in typical deletion subjects when compared to patients with UPD15.