OBJECTIVE The balance of evidence suggests that acromegaly is a risk factor for colonic neoplasia. We have evaluated the prevalence of colonic polyps in acromegalics from Southern Italy and characterized the lymphocyte subsets in the colonic lamina propria in order to analyze differences in the colonic immunological environment. METHODS All the patients and controls were submitted to pancolonoscopy. Ten per-endoscopic biopsies of the intestinal mucosa surrounding polyps were carried out to evaluate lymphocyte subsets. METHODS Fifty acromegalics and 318 sex- and age-matched controls entered this study. Colonic lamina propria lymphocyte subsets were studied in 34 patients and 34 controls. RESULTS Colonic polyps were resected in 23 acromegalics (46%) and 42 controls (13.2%; P < 0.0001); hyperplastic polyps were found in 24% and 6.3%, adenomatous polyps in 22 and 6.9%, (P < 0.01), adenocarcinoma in 2 and 1.2% while synchronous polyps occurred in 18% and 2.5% (P < 0.01), respectively. The number of polyps was significantly correlated with age both in acromegalics (r = 0.422, P < 0.005) and in controls (r = 0.865, P < 0.001). However, polyp prevalence was greater in patients aged below 40 yrs (r.r = 1.9) and in patients with two or more skin tags (r.r = 1.2). A significant decrease of CD20, CD19, CD16, gamma/delta, CD4@leu8- and increase of CD3 and CD4+/leu8+ was found in the lamina propria lymphocyte subsets. CONCLUSIONS The results of this study confirm that acromegalics are at increased risk of colonic polyps compared to the healthy population. The increased prevalence of premalignant polyps, namely the adenomatous type, suggests that acromegalics should undergo a careful screening and follow-up by pancolonoscopy. An impairment of mucosal immune surveillance seems to exist in acromegaly although a causal effect in the polyp formation cannot be ruled out.