Microsomal epoxide hydrolase catalyses the hydrolysis of epoxides to water-soluble trans-dihydrodiols. We studied the expression of the hydrolase in synovial tissue samples from patients with osteoarthritis (n = 20), rheumatoid arthritis (n = 36), ankylosing spondylitis (n = 10) or psoriatic arthritis (n = 15) by use of immunohistochemistry with videodensitometric quantification of staining. Strong immunostaining for microsomal epoxide hydrolase was detected in tunica media of synovial blood vessels and moderate staining in synovial lining cells. Experiments with antibodies against CD68 and CLA suggested that both type A (macrophage-like) and type B lining cells (fibroblast-like synoviocytes) express the hydrolase. In addition, some of the subsynovial fibroblast-like cells, histiocytes and monocytes were intensively stained for microsomal epoxide hydrolase. In general, there were no major differences in the intensity of immunostaining for the hydrolase between the diagnostic groups. The enzyme may be involved in local hydrolysis of epoxide metabolites of endo- and xenobiotics in synovial tissue.