Microsomal epoxide hydrolase is a biotransformation enzyme which is involved in the hydrolysis of various epoxides and epoxide intermediates. In the present study, its distribution was investigated in both normal human tissues and human tumours of different histogenetic origin using immunohistochemical techniques. In normal tissue, epithelial cells were more often and more intensely immunostained than mesenchymal cells. The main epithelial cell types expressing microsomal epoxide hydrolase were hepatocytes, acinus cells of the pancreas, and cells of salivary and adrenal glands. Immunostained cells of mesenchymal origin included monocytes, fibrocytes, fibroblasts, vessel endothelium, muscle cells, and cells of the reproductive system. Three patterns of expression were observed in tumour tissues: (1) moderate or strong in hepatocellular carcinomas, tumours of the adrenal gland, and theca-fibromas of the ovary; (2) inhomogeneous staining pattern of variable intensity in breast cancer, lung cancer, colorectal carcinomas, carcinoid tumours, and some tumours of mesenchymal origin; and (3) no expression in malignant melanomas, malignant lymphomas, and renal carcinomas. These data indicate that microsomal epoxide hydrolase expression is not restricted to tissue of any particular histogenetic origin. Nonetheless, immunohistochemical identification of microsomal epoxide hydrolase may be helpful in some well-defined histological settings, for example, confirmation of hepatocellular carcinoma.