There is substantial evidence for the beneficial effect of screening programmes aimed at reducing mortality from colorectal cancer (CRC). The effect on all-cause mortality, however, may not necessarily be beneficial. In the present study we used the follow-up results 13 years after a flexible sigmoidoscopy screening to evaluate the long-term effects of informing participants about findings at flexible sigmoidoscopy (FS) screening. There were no severe complications and there was no long-term difference in deaths related to whether there had been any mucosal rupture (biopsy or snare resection) or not. As a group, those who attended in 1983 and were informed that they had polyps tended to improve their smoking habits more than those informed that they had no polyps. Similarly, and in spite of more people giving up smoking, the group with polyps had a trend towards a smaller increase in BMI (+0.7 (95% CI 0.2-1.1)) than the polyp-free group (+1.2 (95% CI 0.9-1.6)) (P = 0.07). The observations suggest that flexible sigmoidoscopy screening may face an educational challenge to avoid unfavourable changes in the lifestyle of screenees, an effect that may more than outweigh the beneficial effect of screening.