The rates of collagen biosynthesis and the tissue concentrations of collagen in normal scar, hypertrophic scar and keloid were determined as a function of the duration of the lesions. The rate of collagen synthesis in normal scar was approximately constant between 6 months and 20 years after the initial wounding, but in both hypertrophic scar and keloid the rate was initially approximately twice that in normal scar, and 2-3 years after wounding it fell to approximately the same level as in normal scar. The tissue concentration of collagen appeared to be relatively constant in normal scar with respect to the time elapsed after wounding, but in both types of abnormal scar it was initially somewhat lower than normal scar and then rose to values higher than in normal scar after about 2-3 years. Since similar results were obtained for both types of abnormal scar in terms of the rates of collagen biosynthesis with respect to the time elapsed after wounding and also in terms of the tissue concentrations of collagen, this may indicate that the events in the pathogenesis of both abnormal scar types are similar. The results also raise the possibility that the formation of both types of abnormal scar may occur in two phases: an initial phase characterized by abnormally high rates of collagen biosynthesis and a later phase characterized by essentially normal rates of collagen biosynthesis.