The major histopathological feature of hypertrophic scar lesions is fibrosis, characterized by excessive accumulation of collagen. The purpose of this study was to determine if there is not only increased expression of collagen but also decreased expression of collagenase in hypertrophic scar fibroblasts. We compared the expression of mRNA for alpha 1 (I) and alpha 1 (III) collagen, and collagenase in cultured fibroblasts from different portions of hypertrophic scars and normal dermis. In hypertrophic scar fibroblasts, increased levels of alpha 1 (I) and alpha 1 (III) collagen mRNAs were observed in fibroblasts from the edge and outside of scar tissue, while normal levels were noted in fibroblasts from the centre of this tissue. In contrast, decreased levels of collagenase mRNA were found in the hypertrophic scar fibroblasts. The reductions were: centre (25% of the control) greater than the edge (43% of the control) greater than the outside (84% of the control). The changes in the collagenase mRNA levels of the hypertrophic scar fibroblasts correlated well with decreased collagenolytic activity as determined by the degradation rate of fluorescein isothiocyanate-labelled type I collagen in fibroblast culture supernatant. These results suggest that decreased expression of collagenase in hypertrophic scar fibroblasts may be one possible cause for the excessive accumulation of collagen in the skin lesions of hypertrophic scars.