Human skin collagenase was quantitated by radioimmunoassay in 40 patients with various forms of epidermolysis bullosa to compare levels of the enzyme in blistered and clinically unaffected skin. Immunoreactive human skin collagenase was significantly elevated in the blistered skin of patients with both recessive and dominant forms of dystrophic epidermolysis bullosa (DEB). In addition, patients with generalized recessive DEB manifested a 4-fold increase in collagenase protein in normal-appearing skin, and patients with localized recessive DEB or epidermolysis bullosa letalis showed a 3-t to 3.5-fold elevation in the enzyme. However, patients with dominantly inherited DEB failed to displays a statistically significant increase in immunoreactive collagenase in nonblistered skin. Although it cannot be definitely stated whether the elevated collagenase content in the blistered skin represents a primary or secondary event, such as part of a wound healing response, the demonstration of markedly increased levels of collagenase in normal-appearing skin could, in part, provide an explanation at the molecular level for the formation of blisters in this disease.