The reported frequencies of Gs mutations (gsp mutations) in growth hormone (GH)-secreting pituitary adenomas are variable (ranging from 4·4 to 43%), and the presence of these mutations in the other pituitary adenomas is still a matter of controversy. Previous clinical and biochemical analyses of patients with GH-secreting pituitary adenomas and gsp mutations produced conflicting results and did not demonstrate obvious characteristics. Therefore, we investigated the prevalence of gsp mutations in Korean patients with pituitary adenomas and elucidated the characteristics of these patients. Forty-four GHsecreting adenomas, 7 prolactin (PRL)-secreting adenomas and 32 clinically non-functioning adenomas were examined for the presence of point mutations in codon 201 and 227 of the Gs gene using a nested PCR and direct sequencing of DNA extracted from fresh tissue or paraffinembedded pituitary adenoma samples. Seven of the 44 GH-secreting pituitary adenomas had point mutations at codon 201 or 227; of these, five mutations were in codon 201 and two were in codon 227. In patients with gsp mutations, mean tumor size was significantly smaller than in patients without gsp mutations (15·9 8·7 mm vs. 24·9 14·9 mm, P<0·05). Age, sex, basal GH levels, GH response to oral glucose loading, GH response to octreotide and surgical outcome were not different in the two groups. One of the 32 clinically non-functioning pituitary adenomas had a point mutation at codon 201; none of the seven prolactinomas had these mutations. These results show that gsp mutations are not rare in Korean acromegalic patients and mean tumor size is significantly smaller in acromegalic patients with gsp mutations. Our results also confirm the low frequency of gsp mutations in clinically non-functioning pituitary adenomas and the absence of gsp mutations in prolactinoma.