Twelve acromegalic patients were treated (mean +/- SD) 26 +/- 15 months with daily doses of 440 +/- 330 micrograms of the somatostatin analogue octreotide acetate (SMS 201-995, Sandostatin). The levels of somatomedin-C (Sm-C) decreased by 63% from 8.1 +/- 7.7 U/ml to 3.0 +/- 1.3 U/ml. Before starting therapy a long oral glucose tolerance test (oGTT) and a TRH test were performed both without and after s.c. injection of 100 micrograms octreotide. Under long-term treatment with octreotide four of twelve patients reached normal Sm-C-values. The GH levels of all of these patients were continuously suppressed to less than 2 ng/dl in an oGTT after a test dose of 100 micrograms octreotide s.c. till the end of the test (5 1/2 hours after octreotide injection). The other eight patients had a relief of acromegalic symptoms and five had a decrease of their Sm-C-levels, but none of them reached normal Sm-C-values. None of these patients had a continuous suppression of GH after a test dose of octreotide in an oGTT. Hyperprolactinemia (n = 4) was observed only in those patients with an insufficient response to octreotide. The GH-response to TRH showed neither without nor after injection of octreotide a correlation with the results of long-term treatment. Thus it is concluded that GH-suppression in a long oGTT after administration of a test dose of 100 micrograms octreotide acetate s.c. allows to identify those acromegalic patients who will benefit from long-term treatment with the somatostatin analogue octreotide acetate.