Claudins are integral membrane proteins involved in the structure of the tight junctions found in epithelial and endothelial cells. This study evaluated the expression of claudin-5 in 67 hyperplastic and neoplastic lesions of canine hepatoid glands. These included normal hepatoid glands (n = 10), nodular hyperplasia (n = 10), adenomas (n = 12), epitheliomas (n = 15), differentiated carcinomas (n = 15) and anaplastic carcinomas (n = 15). There was intense lateral membrane expression of claudin-5 on epithelial cells from normal hepatoid glands, nodular hyperplasia and adenomas, but expression was weaker in hepatoid gland epitheliomas. Basal reserve cells from normal glands, nodular hyperplasia, adenomas and epitheliomas never expressed claudin-5. There was membrane-bound immunoreactivity for claudin-5 in selected areas of the epitheliomas where the cells exhibited typical hepatoid features. The weak expression of claudin-5 molecule in epitheliomas may nevertheless lead to cellular disorientation, detachment and invasion. Claudin-5 expression seemed to be helpful in distinguishing poorly differentiated carcinomas, differentiated carcinomas and epitheliomas of the hepatoid glands. Increased claudin-5 expression by invasive anaplastic carcinomas may facilitate invasion and metastasis through the activation of matrix metalloproteinases.