Abstract. To study GH response to the long-acting somatostatin analogue, we treated 11 actively acromegalic patients with octreotide (Sandostatin),100 µg, sc, tid, for six months. Their endocrinological outcomes and clinical improvements varied. The 11-h GH secretory profiles on pretreatment day confirmed the hypersecretion of GH in all patients. Three hours after the first dose of octreotide, serum GH declined rapidly to levels below 5 ng/ml in all but two patients who failed to normalize their serum GH. In spite of the subsequent doses, there was no further suppression in serum GH. Drug resistance with GH rebound developed in some patients after three months of continued treatment.The paradoxical serum GH rises in response to oral glucose or iv TRH detected before the treatment in all patients attenuated or disappeared after the 6-month octreotide therapy; an exceptional case was one of the above-mentioned two patients, whose serum GH was stimulated more than before by glucose and TRH at the end of therapy. Serum insulin-like growth factor I (IGF-I) levels of all patients showed a significant reduction after 6-month treatment, but their mean values remained abnormally high. There were no intolerable adverse side effects; some patients, however, experienced pain at the injection site, passage of loose stool, and incidence of new gall stone or intrahepatic lesions on octreotide therapy. We concluded that octreotide was a useful long-term adjunctive therapeutic agent for patients with active acromegaly, but that a high degree of response heterogeneity including total refractoriness would be expected.