A new variant of multisystemic lipid storage myopathy (type 3) has been identified. Human cultured fibroblasts present a major triacylglycerol storage whereas other neutral lipids and phospholipids are in the normal range. When feeding the cells in the presence of radiolabelled oleic acid we observed an accumulation of radiolabelled triacylglycerols demonstrating the endogenous biosynthesis of the stored triacylglycerols. After a 72-hr chase period, no degradation of radiolabelled triacylglycerols was observed. Histochemical examination of multisystemic lipid storage myopathy skin fibroblasts showed a massive accumulation of neutral lipids (stained by the fluorescent probe Nile Red) in cells grown in medium supplemented with 10% fetal calf serum. These cytoplasmic vacuoles were not obviously membrane-surrounded as shown by electron microscopy.