The aim of the study was to evaluate the impact of the long acting somatostatin analogue (aSS, lanreotide) for the serum GH and IGF‑I concentration, pituitary tumor volume and its invasiveness. The results of pharmacological aSS treatment in 17 patients (11 females and 6 males, mean age 43.–35 years, range: 32–64 years) were analyzed. GH, IGF‑1 levels, tumor volume and its invasiveness were measured. Mean initial serum GH and IGF‑1 levels were 20.506 ng/ml and 864.824 ng/ml, respectively, while after the treatment they decreased to 7.259 ng/ml and 544.765 ng/ml, respectively. Mean initial pituitary tumor volume was 3332.765 mm3 and decreased to 2345.206 mm3, which was statistically significant. None of the patients showed invasive lesions caused by the pituitary tumor during the treatment. The results demonstrated that aSS significantly decreases serum level of GH and IGF‑1 and shrinks pituitary tumor. The authors did not observe any impact of the treatment on the pituitary tumor invasiveness.