First trimester prenatal diagnosis was offered to the mother of a child affected by severe haemolytic anaemia due to glucose phosphate isomerase (GPI) deficiency. The mutant enzyme was characterized by an increased thermal lability. Both parents had 50 per cent normal red cell GPI activity. We have shown that the homozygous and heterozygous genotypes can be clearly distinguished from each other and controls by combinations of the measurement of enzyme activity and enzyme thermal lability. Examination of trophoblast cells obtained at 9 weeks of gestation led to the diagnosis of a GPI heterozygous fetus. The result was confirmed by analysis on uncultured and cultured amniotic fluid cells sampled at 16 weeks and by red blood cell studies of the healthy newborn. Prenatal diagnosis of GPI deficiency is indicated in families with previous cases resulting in severe haemolysis and mainly with the conservative view of arranging appropriate therapeutic measures for affected fetuses.