Red cell pyruvate kinase (PK) deficiency is the most frequent enzyme abnormality of glycolysis causing chronic non-spherocytic haemolytic anaemia. The disease is transmitted as an autosomal recessive trait, clinical symptoms usually occurring in compound heterozygotes for two mutant alleles and in homozygotes. The severity of haemolysis is highly variable, ranging from very mild or fully compensated forms to life-threatening neonatal anaemia necessitating exchange transfusions. Erythrocyte PK is synthesised under the control of the PK-LR gene located on chromosome 1. One hundred eighty different mutations in PK-LR gene, mostly missense, have been so far reported associated to PK deficiency. First attempts to delineate the genotype-phenotype association were mainly based on the analysis of the enzyme's three-dimensional structure and the observation of the few homozygous patients. More recently, the comparison of the recombinant mutants of human red cell PK with the wild-type enzyme has enabled the effects of amino acid replacements on the enzyme molecular properties to be determined. However, the clinical manifestations of red cell enzyme defects are not merely dependent on the molecular properties of the mutant protein but rather reflect the complex interactions of additional factors, including genetic background, concomitant functional polymorphisms of other enzymes, posttranslational or epigenetic modifications, ineffective erythropoiesis and differences in splenic function.