This paper reports the prenatal diagnosis of HMG CoA lyase deficiency at 16 weeks' gestation by direct chemical analysis of cell-free amniotic fluid and by measurement of HMG CoA lyase activity in cultured amniocytes. Termination of an affected fetus allowed study of chorionic villus tissue, the results providing the basis for future first trimester prenatal diagnoses of this condition. An abstract report of this work has appeared elsewhere.