To uncover pathogenic deep intronic variants in patients with colorectal adenomatous polyposis, in whom no germline mutation in the APC or MUTYH genes can be identified by routine diagnostics, we performed a systematic APC messenger RNA analysis in 125 unrelated mutation-negative cases. Overall, we identified aberrant transcripts in 8% of the patients (familial cases 30%; early-onset manifestation 21%). In eight of them, two different out-of-frame pseudoexons were found consisting of a 167-bp insertion from intron 4 in five families with a shared founder haplotype and a 83-bp insertion from intron 10 in three patients. The pseudoexon formation was caused by three different heterozygous germline mutations, which are supposed to activate cryptic splice sites. In conclusion, a few deep intronic mutations contribute substantially to the APC mutation spectrum. Complementary DNA analysis and/or target sequencing of intronic regions should be considered as an additional mutation discovery approach in polyposis patients. Hum Mutat 33:1045–1050, 2012. © 2012 Wiley Periodicals, Inc.