BACKGROUND AND OBJECTIVE: It was the aim of this study to compare the sensitivity and specificity of low-dose CT colonography (CTC) with that of optical colonoscopy (OC) in asymptomatic patients undergoing these tests in a screening program for colonic cancer. PATIENTS AND METHODS: 58 patients (mean age 62.6 years) were included. They underwent low dose CTC and, immediately afterwards, colonoscopy. The colonoscopists were unaware of the CTC findings. A 'second look” was performed if a lesion seen in CTC had been missed in the first colonoscopy. RESULTS: A total of 150 lesions were detected and histologically confirmed. 136 were found to be polypoid lesions, classified as either hyperplastic polyps (n = 66) or polyps with intraepithelial neoplasia (n = 70). In the per-patient analysis only 22.4 % of patients had no polypoid lesion, 27.6 % had at least one hyperplastic and 50.0 % had at least one adenomatous lesion. Sensitivity for adenomas of all size categories was calculated 55.7 % for CTC and 92.9 % for OC. This marked difference (both for the detection of individual lesions and the per-patient analyses) does not reach significance in the two-sided McNemar test. CONCLUSIONS: There was a high prevalence of lesions with intraepithelial neoplasia in this screening group. OC had a higher sensitivity than CTC in the detection of lesions smaller than 10 mm.