Objective To investigate the cause of excessive collagen accumulation in keloid tissue. Methods The ultrastructure of keloid was observed by transmission electron microscope. New formed collagen in keloid was localized with ABC immunohistochemical staining. Type I procollagen mRNA level in keloid tissue was determined by dot blot hybridization using human pro-al (I)collagenspecific cDNA probe. Results Numerous fibroblasts with abundant, well developed rough endoplasmic reticulum were exhibited in the ultrastructure of keloid. The type I procollagen mRNA levels were significantly increased in kreloid tissue. Immunohistochemical staining showed increased expression of new formed, type I procollagen in keloid tissue. Conclusion the fibroblasts are activated in collagen synthesis in active keloid. The enhanced collagen synthesis by fibroblasts is a critical factor leading to the overabundant collagen accumulation in keloid.