We treated four patients with acromegaly for 8 to 24 weeks with SMS 201-995, the long-acting somatostatin analogue, in dosages of 100 to 300 micrograms a day given subcutaneously. A rapid amelioration of the clinical signs and symptoms and near normalization of laboratory test results occurred in all patients. Mean plasma growth hormone concentrations (+/- S.E.M.), as measured over 24 hours, fell from an initial value of 57 +/- 18 micrograms per liter to 7.5 +/- 2 micrograms per liter at the end of the investigational period. Likewise, levels of plasma somatomedin-C, which were originally elevated in all patients, dropped to the normal or nearly normal range. The suppression of insulin secretion and the resulting hyperglycemia that were observed at the beginning of treatment became less marked as therapy progressed. There was evidence of slight tumor shrinkage in three of the subjects. No side effects were recorded throughout the treatment period. These preliminary results suggest that SMS 201-995 represents an additional option for the management of acromegaly, especially in patients who do not benefit sufficiently from surgery or radiotherapy and do not respond well to treatment with dopaminergic drugs.