Two children with nonspherocytic anaemia and pyruvate kinase deficiency had their diagnosis delayed due to normal initial estimations of enzyme activity. Repeated analyses involving other glycolytic enzymes documented an increased glucose-6-phosphate dehydrogenase activity as a function of high reticulocyte counts. The pyruvate kinase activity showed subnormal values. By simple comparison of both enzyme activities a true pyruvate kinase activity of below 50% was estimated thus rendering the diagnosis of pyruvate kinase deficiency highly probable. Analyses of more than one glycolytic enzyme should be performed in young children with otherwise unexplained haemolysis and associated high reticulocyte counts.