We studied two unrelated patients with autosomal recessive multisystem triglyceride storage disease. Cultured fibroblasts accumulated 10 times more triglyceride than controls under glycerol or palmitate feeding. Mutant fibroblasts could not degrade accumulated triglycerides of endogenous origin, but normally degraded endogenously synthesized phospholipids. When the cells were fed with exogenous olein, triglyceride catabolism was in the normal range. Oxidation of long-chain, medium-chain, and short-chain fatty acids was normal, and the activities of acidic, neutral, and alkaline lipase in cell extracts were normal. The disease seems to be due to a specific impairment in the degradation of triglycerides synthesized endogenously.