Gingival overgrowth tissues have thickened connective tissue stroma, sometimes accompanied by the increased presence of collagen fibers, thickened epithelia, and elongated rete pegs. We have previously shown that expression of CCN2, also known as connective tissue growth factor (CTGF), correlates positively with the degree of gingival fibrosis, and that markers of epithelial to mesenchymal transition (EMT) are characteristic of all drug-induced forms of gingival overgrowth. Here we experimentally evaluate whether increased degradation of the basement membrane and apparent invasion of the underlying stroma by epithelial cells could be observed in human gingival overgrowth tissues. Tissues from 20 different individuals with human gingival overgrowth and 15 non-overgrowth samples were evaluated by histological analyses and by immunohistochemistry assays of basement membrane proteins. The results demonstrate that there are significantly higher numbers of basement membrane discontinuities in overgrowth tissues, sometimes containing epithelial-like cells. Disrupted basal membrane structure in gingival overgrowth tissues is accompanied by a discontinuous collagen type IV expression pattern and decreased laminin 5. These findings provide new additional support for the hypothesis that epithelial plasticity and EMT promote gingival overgrowth, resulting in compromised basal membrane structure and increased interactions between epithelial and connective tissue layers that contribute to fibrotic pathology.