Two brothers with a recently described inborn error of metabolism characterized by glyceroluria, hyperglycerolemia, and generalized glycerol kinase deficiency had moderate psychomotor retardation, spasticity, growth failure, a nonspecific myopathy, osteoporosis, and adrenal insufficiency. Glycerol kinase activity in leukocytes and cultured fibroblasts was less than 5% of control values. Hepatic and renal tissue obtained at autopsy in one patient had similarly low enzyme activity. Thus the deficiency of glycerol kinase in these patients appears to be generalized and heritable, though the relationship of the clinical phenotype to the enzymatic defect is not yet established.