Follicular carcinoma (FC) of the thyroid is distinguished from follicular adenoma (FA) by confirmation of invasion or metastasis. However, it is still unknown how FC acquires the potential for invasion or metastasis. Twenty early FC (pT1 and pT2) were analyzed for immunohistochemical E-cadherin (E-CD) expression. Four of four (100%) widely invasive FC showed reduced E-CD expression, whereas only two of 16 minimally invasive FC showed reduced expression. In most of the minimally invasive FC, both E-CD and beta-catenin were expressed on cell-cell boundaries, even in areas where capsular invasion occurred. These results suggest that the mechanism of invasion may be different between widely invasive FC and minimally invasive FC, and that E-CD is probably responsible for the invasion of widely invasive FC. Analysis of methylation of the E-CD gene promoter using the methylation-specific polymerase chain reaction (MSP) method revealed no methylated alleles in the DNA from FC.