A second case is reported in which Lesch-Nyhan syndrome was diagnosed prenatally, pregnancy terminated and the enzyme deficiency in fetal tissues confirmed. A micromethod for H-PRT determination, which could make earlier prenatal diagnosis possible, has been applied to demonstrate the enzyme deficiency in cultured amniotic cells. In contrast to an earlier report, measurable amounts of H-PRT activity were found in the tissues of the affected fetus. The possibility of using the ratio of uric acid to creatinine in amniotic fluid as additional evidence in prenatal diagnosis of the Lesch-Nyhan syndrome is discussed.