A double-blind study of atypia in colorectal adenomatous polyps was done with autopsy material from Cali, Colombia (low colon risk), São Paulo, Brazil (intermediate risk), and New Orleans, Louisiana (high risk). An interobserver reproducibility of 82% was obtained. The prevalence of atypia in polyps increased with the cancer risk and also with the size of polyps, but some exceptions to the rule were found. The frequency of atypia increased steadily with age in Cali, but it plateaued after age 50 in the higher risk populations.