This single-center open sequential study aimed at comparing the efficacy of a 6-month treatment with lanreotide (LAN) (60-90 mg/month i.m.), to that of octreotide (OCT) (0.3-0.6 mg/day s.c.) in 45 patients with active acromegaly (GH, 63.2+/-12.1 ng/ml, IGF-I, 757+/-67.1 ng/ml). After 6 months of OCT treatment, safe GH (fasting <2.5, glucose suppressed <1 ng/ml) and IGF-I (normalized for age) levels were achieved in 23 patients. After treatment withdrawal, GH levels significantly increased in all patients, though remaining slightly lower than pre-OCT therapy (39.2+/-5.8 ng/ml) while plasma IGF-I levels were unchanged (654+/-59.4 ng/ml). After 6 months of LAN treatment, safe GH and IGF-I levels were achieved in 26 patients (57.7%). After OCT or LAN treatments, no significant difference was found between nadir GH (6+/-1 vs 5.9+/-1.1 ng/ml) and IGF-I levels (281+/-23.3 vs 262+/-20.6 ng/ml). Four out of the 20 patients poorly responsive to OCT achieved safe GH and IGF-I levels after LAN treatment. Among the 20 non-operated patients, a significant tumor shrinkage was documented by CT and/or MRI in 5 patients after OCT and in 1 patient after LAN treatment. All patients referred a notable improvement of soft tissue swelling, arthralgia, headache and weakness, both after OCT and LAN treatments. During the first days of OCT treatment, abdominal discomfort was referred by 12 patients and steatorrhea by 5 patients: side effects disappeared spontaneously in 6 cases while during treatment with pancreatic enzymes in the remaining ones. After the first injections of LAN, abdominal discomfort was referred by 10 patients and steatorrhea by 2 of them. No difference in the prevalence of both early and late side effects was noted after treatment with OCT and LAN (chi2, 0.49). The majority of these poorly tolerant patients had side effects with both compounds. During LAN treatment, side effects were mild and spontaneously disappeared but recurred after the injection of the drug in six patients. Gallstones were detected in one patient during OCT and in another during LAN, sludge was noted in 6 patients after OCT and in 2 after LAN treatment. In conclusion, the treatment with LAN allowed to achieve safe GH and IGF-I levels in 57.7% of acromegalics with an excellent patients' compliance. LAN treatment possessed similar efficacy and caused side effects with a similar incidence of OCT treatment. The recurrence of side effects after LAN injection suggests the necessity of a careful monitoring of adverse reactions.