Ten cases of endoscopically removed colorectal polypoid tumors exhibiting lobular growth patterns in the submucosa without prominent desmoplastic changes in the interstitium were investigated using serial sections, and four cases were confirmed to be pseudoinvasion. The growth pattern of these four cases (pseudoinvasive tumors) was morphologically compared with the other six tumors (microinvasive tumors) in which obviously infiltrating foci were seen in minimal ranges. In the pseudoinvasive tumors, intramucosal tumor tissue spread into the submucosa through the narrow gap of the muscularis mucosae and formed a lobulated nodule larger than the gap of the muscularis mucosae. This suggested that squeezing of the herniated tumor tissue by muscularis mucosae at the gap was crucial to forming a typical feature of pseudoinvasion. The maximum diameters of the gap of the muscularis mucosae (G) and the submucosal tumor nodule (N) were measured under a microscope and compared between both groups. The mean N/G ratio of the pseudoinvasive tumors (1.73 ± 0.46) indicated a significantly higher value than that of the microinvasive tumors (1.04 ± 0.06; P < 0.01). The N/G ratio could be one of the indices used to distinguish a pseudoinvasive tumor from a microinvasive tumor in colorectal polypoid tumors.