In 6 cases of Dupuytren's disease and 1 of Ledderhose's disease, the nodules of the palmar and plantar aponeurosis were examined by light and electron microscopy. The cells composing these nodules, presumably fibroblasts, showed three significant ultrastructural features: (1) a fibrillar system similar to that of smooth muscle cells; (2) nuclear deformations such as are found in contracted cells, the severest being recognizable by light microscopy (cross-banded nuclei); (3) cell-to-cell and cell-to-stroma attachments. Based on these data and on recent information about the biology of the fibroblasts, it is suggested that these cells are fibroblasts that have modulated into contractile cells (myofibroblasts), and that their contraction plays a role in the pathogenesis of the contracture observed clinically.