Myofibroblasts and the contraction of split skin-graft donor sites were studied in 18 patients. For each patient five samples were studied, taken on days 0, 14, 21, 28, and 35. On each occasion the extensions and densities of myofibroblasts were calculated. After the initial measurement, the subsequent extensions were expressed in percentages of the first, resulting in mean measurements of 100, 95, 97, 99, and 99%, respectively. The myofibroblasts in 10 dermic fields were counted under light microscopy at a magnification of 1000 marked with anti-alpha-smooth muscle actin antibodies using immunoperoxidase, resulting in the means of 0.2, 3.8, 1.3, 1.3, and 0.4, respectively. The contraction and the increase in density of myofibroblasts were transitory and significant in the samples measured on day 14, but in sample 35 neither of the variables was significantly different from the initial sample. These variables evolved concurrently, corroborating the hypothesis that myofibroblasts are responsible for the contraction.