To determine the expression of two isoforms of the growth hormone (GH) receptor (GHR), which differ by the presence (GHR3+) or absence (GHR3-) of exon 3, and their correlation with circulating GH and insulin-like growth factor I (IGF-I) in normal subjects and in acromegalic patients.The expression of GHR isoforms was determined by reverse-transcriptase polymerase chain reaction in lymphocytes from 12 normal subjects and from 11 patients with acromegaly. The levels of GHR mRNA were normalized to those of beta-actin, and ratios were calculated to assess the relative levels of expression.All samples showed expression of both GHR isoforms, but the expression of GHR3+ and GHR3- was similar in acromegalic patients (6.0+/-1.7 vs. 8.3+/-2.0%, mean +/- SE). In contrast, in healthy subjects, GHR3- was the predominant isoform (11.8+/-3.0 vs. 5.1+/-0.68%; p