The clinical consequences of growth hormone (GH) deficiency (GHD) in adults have not been defined. Standard methods of measuring GH reserve in children may not be reliable in adults. In addition, obesity in normal adults diminishes GH responsiveness to provocative stimuli; this inhibition of GH release is reversed with pyridostigmine (PD). We investigated the use of GH-releasing hormone (GHRH) as a method to assess pituitary GH secretory reserve (as defined by peak GH response to GHRH) in both non-obese and obese (ie, > 115% ideal body mass index [BMI]) adults with hypothalamic-pituitary tumors. Nine non-obese patients (NOP) and 10 obese patients (OP) were studied with 11 non-obese controls (NOC) and 10 obese controls (OC). All study groups received GHRH (1 microgram/kg intravenous bolus) with blood sampling at -15, 0, 15, 30, 45, 60, 75, and 90 minutes. OC and OP received 120 mg PD orally 1 hour before GHRH injection. Mean serum GH responses in NOC and OC were significantly higher (P < .05) than those in NOP and OP, respectively, 15 minutes after GHRH injection, and remained so throughout the time-course of the test. Mean +/- standard error of the mean (SEM) peak GH level (microgram/L) was lower in NOP than in NOC (5.1 +/- 1.6 v 21.2 +/- 4.4, P < .01) and lower in OP than in OC (4.6 +/- 1.8 v 15.5 +/- 2.2, P < .01). Mean +/- SEM peak GH level was also lower in NOP than in OC (5.1 +/- 1.6 v 15.5 +/- 2.2 micrograms/L, P < .01).(ABSTRACT TRUNCATED AT 250 WORDS)