We present a congenital plexiform cellular tumor with high mitotic activities arising in the right thigh of a 3-days-old infant. This subcutaneous tumor measured 6.5 x 4.5 cm in diameter with multinodular, whitish, elastic cut surface. Microscopically, the tumor was composed of fascicles of closely packed uniform spindle cells with frequent nuclear palisadings. The most remarkable finding was frequent mitoses(4-5/10 HPF). The tumor cells were strongly positive for S-100 protein, myelin basic protein and vimentin, and weakly positive for Leu 7. On electron microscopic examination, the spindle cells were found to be surrounded by continuous basal lamina and had interlocking long cytoplasmic processes. Although the prognosis of this tumor is difficult to predict, aggressive behavior such as recurrences may be suggested, but it is less likely to be metastatic. Further accumulation of similar unusual cases may be helpful in evaluation of its biologic behavior.