AIMS: To examine the expression of MUC1 and MUC2 apomucins and distribution of MUC phenotypes (MUC2+/ MUC-, MUC2+/MUC1+, MUC2-/ MUC1+, MUC2-/MUC1-) in colorectal tubular adenomas in order to compare the distribution of phenotypes in flat and polypoid adenomas. METHODS: Endoscopically resected specimens of 35 flat and 15 polypoid tubular adenomas measuring less than 10 mm were examined and compared for the expression of MUC1 (MUSE11) and MUC2 (CCP58) and combined MUC phenotype distribution using conventional immunohistochemistry. RESULTS: There was no significant difference between flat and polypoid adenomas in their expression of MUC1 and MUC2 and the MUC phenotype distribution when stratified by grade of histological atypia. Adenomas with low grade atypia showed more extensive MUC2 expression than MUC1 (MUC2+/MUC1-phenotype). Expression of MUC1 was more extensive in adenomas with high grade atypia and the majority displayed either MUC2+/ MUC1+ or MUC2-/MUC1+ phenotypes. CONCLUSIONS: MUC2/MUC1 phenotypes were similar in flat and polypoid adenomas when stratified by grade of atypia. High grade atypia was characterised by reduced MUC2 and increased MUC1 expression in both types of adenoma. The phenotype MUC2-/MUC1+ occurs in tubular adenomas and cannot be a specific marker for de novo colorectal cancer.