In vitro growth kinetics of fibroblasts derived from normal skin and hypertrophic scar were performed using continuous 3H-thymidine labeling method. In fibroblasts derived from normal skin, aging of the donor affects cell growth mainly by growth fraction (GF), but not labeling index (LI) and DNA synthetic time (Ts). When hypertrophic scar-derived fibroblasts are compared with normal fibroblasts, they showed a shorter Ts and lower LI and GF. This result suggests that in hypertrophic scar a small number of fibroblasts proliferate more actively, but most fibroblasts are non-growing cells.