OBJECTIVE It is unclear whether the blunted GH secretion in Prader-Willi Syndrome (PWS) is a true deficiency, or merely secondary to obesity. We have investigated the role of obesity in the blunted GH secretion in PWS. METHODS We studied the GH response to a combined administration of GHRH (1 microgram/kg i.v. at 0 min) and pyridostigmine (PD) (60 and 120 mg by mouth for children and adults, respectively, at time -60 min), as well as the baseline IGF-I levels, in a group of patients with PWS. Two different control groups were studied with GHRH + PD using the same doses and methods as above: prepubertal and pubertal obese subjects, and prepubertal short normal children. Moreover, in 14 patients with PWS and in the group of short normals the GH response to at least two stimulation tests (insulin tolerance test, clonidine, L-dopa, arginine) had been previously determined. METHODS Twenty-two PWS patients (10 males and 12 females), 21 with essential obesity (11 males and 10 females), and eight short normal children (4 males and 4 females) were studied after obtaining informed consent. METHODS Blood samples were taken at -60, -30 and 0 min and then 15, 30, 45, 60, 90 and 120 min after GHRH administration. Serum GH was measured in duplicate by IRMA, and IGF-I by RIA after acid ethanol extraction. Statistical analysis was performed by t-test for unpaired data, and analysis of variance for parametric or nonparametric data, where appropriate. RESULTS The GH response to GHRH + PD was significantly lower in PWS patients (AUC: mean +/- SE: 599 +/- 99 micrograms/l/h) if compared with either short normal children (3294 +/- 461 micrograms/l/h: P < 0.0001) or obese subjects (1445 +/- 210 micrograms/l/h: P < 0.005). Low IGF-I concentrations were found in all PWS patients, so that PWS group had mean IGF-I levels significantly lower than the other groups. CONCLUSIONS Our results showed that subjects with PWS had a reduced GH responsiveness to GHRH + PD associated with subnormal IGF-I levels. These findings suggested that short stature in PWS may be at least partially correlated to the presence of GH deficiency, and that impaired GH secretion is not secondary to obesity.