Nine patients with Zollinger-Ellison syndrome were treated with octreotide acetate (100 micrograms delivered subcutaneously three times daily) and followed up for 1 to 48 months. Serum gastrin levels were obtained at predetermined intervals. All patients had elevated baseline fasting gastrin levels (greater than 150 ng/L [greater than 150 pg/mL]). One month after administration of octreotide, gastrin levels were in the reference range for five (62%) of eight patients, and a mean gastrin suppression rate of 76% was achieved (ie, values were a mean of 76% less than baseline values). One year after administration of octreotide, five (71%) of seven evaluable patients had gastrin levels of less than 200 ng/L (200 pg/mL), and the mean gastrin suppression rate was more than 80% for these seven patients. During the second year, control at these levels was maintained in four patients; one patient continued to have controlled levels for 42 months. Complete symptomatic response occurred in seven patients (78%), and partial response in two patients (22%). All six patients with diarrhea before treatment were cured of it. Octreotide acetate provides efficacious long-term suppression of elevated gastrin levels and excellent symptomatic relief in patients with Zollinger-Ellison syndrome.