OBJECTIVE It has been suggested that patients with acromegaly may be at risk of developing colorectal carcinoma. In order to clarify this issue, we have evaluated the prevalence of carcinoma, premalignant tubulovillous adenomas and hyperplastic colonic polyps in a large cohort of patients with acromegaly. METHODS Prospective colonoscopic examination by a single operator. METHODS One hundred and twenty-nine patients with biochemically proven acromegaly. RESULTS At least one lesion was visualized in 63 patients. Adenocarcinoma was present in six patients (5%), but only two had symptoms; all lesions were endoscopically obvious. Compared with a normal group, the odds ratio of colorectal cancer is increased at 13.5 (95% confidence intervals (c.i.) 3.1-75). One or more tubulovillous adenoma was found in 34 patients (26%) and this prevalence was age-dependent, occurring in 39% of patients aged 70 years or over. Comparing the prevalence of left-sided colonic adenomas with that in a normal group, there is a higher prevalence among patients over 49 years with an odds ratio of 4.2 (95% c.i. 2.5-6.8). Patients with acromegaly who had an adenoma were significantly older than unaffected patients (61.9 vs 54.1 years; P < 0.001) but had similar GH and IGF-1 levels and duration of disease. CONCLUSIONS Patients with acromegaly have an increased risk of developing colorectal cancer and a significantly higher prevalence of tubulovillous adenomas compared with normal subjects. Routine surveillance colonoscopy is indicated in this group of patients.