Seventeen patients (8 women and 9 men) resistant to all other forms of therapy were treated with the somatostatin analogue SMS 201-995 (octreotide, Sandostatin). The duration of treatment ranged from 1 to 5 years. Mean GH levels of only 4 patients were suppressed under 5 micrograms/L during an 8 h serum profile with the standard dose of 0.1 mg 2 or 3 times daily. This standard dose suppressed mean GH levels in 10 other patients more than 50% of baseline, but for optimal effect higher doses up to 1.5 mg, 4 daily injections or continuous subcutaneous infusion (CSI) were needed. Octreotide had no influence on GH secretion in 3 patients. Suppression of mean GH levels under 5 micrograms/L was achieved in 10 patients. Normalization of insulin-like growth factor I (IGF-I) occurred in only 5 patients. Altogether, therapy with SMS 201-995 reduced GH levels from 23.8 +/- 32.2 micrograms/L (mean +/- SD) to 6.7 +/- 5.0 micrograms/L by 71.8% and IGF-I levels from 7.9 +/- 3.1 U/ml to 3.2 +/- 1.6 U/ml by 59.5%. We conclude that 1) treatment with SMS 201-995 in patients resistant to other forms of therapy may be less successful than previously reported for heterogenous groups of patients; 2) the dose regimen must be adapted to the individual patient for optimal effect and most of our patients needed higher doses than 300 micrograms daily; 3) 4 or maybe more daily injections or CSI seem to be most effective; and 4) in a minority of patients SMS has no influence on GH-secretion.