Aims Despite almost pathognomonic status of ectopic crypt foci (ECF) in the diagnosis of traditional serrated adenoma (TSA), there are few systematic studies on their prevalence in other types of colon polyps or in adenomas adjacent to colorectal cancer (CRC).

Methods We calculated ECF in all the polyps (n=922) removed in the colonoscopy in Oulu University Hospital in 2001. Moreover, to study ECF in precursor lesions next to CRCs, we re-examined a previously described cohort of 148 CRCs.

Results ECF were seen in 53 (5.7%) polyps representing 28 (6.5%) tubular adenomas (TAs), 14 (53.8%) tubulovillous adenomas (TVAs), 2 (100.0%) villous adenomas (VAs) and 9 (100.0%) TSAs. In all TSAs and VAs, the density of ECF was higher than in TVAs and TAs. An adjacent precursor lesion was recognised in 28 of 148 (18.9%) CRCs. Twenty-four (85.7%) of these contained ECF.

Conclusions ECF can most frequently be observed in TSAs but also in many TVAs, VAs and TAs, reflecting a histological overlap between serrated and conventional polyps. Especially, precursor lesions adjacent to CRC frequently contain ECF.