The long-acting somatostatin analogue octreotide is used either as an adjuvant or primary therapy to lower growth hormone (GH) levels in patients with acromegaly and may also induce pituitary tumor shrinkage. We performed a meta-analysis to accurately assess the effect of octreotide on pituitary tumor shrinkage. A computerized Medline and Embase search was undertaken to identify potentially eligible studies. Eligibility criteria included treatment with octreotide, availability of numerical metrics on tumor shrinkage and clear definition of a clinically relevant reduction in tumor size. Primary endpoints included the proportion of patients with tumor shrinkage and mean percentage reduction in tumor volume. The electronic search identified 2202 articles. Of these, 41 studies fulfilling the eligibility criteria were selected for data extraction and analysis. In total, 1685 patients were included, ranging from 6 to 189 patients per trial. For the analysis of the effect of octreotide on pituitary tumor shrinkage a random effect model was used to account for differences in both effect size and sampling error. Octreotide was shown to induce tumor shrinkage in 53.0% 95% CI: 45.0%-61.0% of treated patients. In patients treated with the LAR formulation of octreotide, this increased to 66.0%, 95% CI: 57.0%-74.0%). In the nine studies in which tumor shrinkage was quantified, the overall weighted mean percentage reduction in tumor size was 37.4% 95% CI: 22.4%-52.4%, rising to 50.6% 95% CI: 42.7%-58.4% with octreotide LAR. Most trials examined were open-label and had no control group. Octreotide LAR induces clinically relevant tumor shrinkage in more than half of patients with acromegaly.