The nature of the enzymatic defect is now well established in ten inherited disorders of lipid metabolism. This information has provided for the development of facile, sensitive tests using readily available materials such as washed leukocytes or cultured skin fibroblasts for the diagnosis of these disorders; the detection of heterozygous carriers of these traits; and the monitoring of pregnancies at risk for any of these conditions. Recent investigations of enzyme replacement therapy have shown great promise for the treatment of patients with Fabry's disease and those with the adult form of Gaucher's disease. Additional procedures must be developed for successful enzyme replacement in patients where the central nervous system is damaged by the accumulation of lipids.