The pathogenesis of multiple symmetric lipomatosis (MSL) is as yet unknown; however, some studies seem to indicate the neoplastic nature of this lesion. In this study we have examined the ultrastructural features of the adipocyte precursors of patients with MSL. The cells were isolated by collagenase, cultured, and then examined at the moment of their confluence in a monolayer. Their ultrastructural features were compared with those of cells of the abdominal subcutaneous adipose tissue taken from patients with nonsystemic, surgically treated illnesses. When examined immediately after isolation, the cells from normal tissue did not show significant differences from MSL cells. In culture, however, MSL grew more quickly into a monolayer and showed numerous nuclear pockets and cytoplasmic microfilaments, which were not seen in cells from normal tissue. These differences appear compatible with the neoplastic nature of the MSL adipocyte precursors.