Eleven new cases of red cell pyruvate kinase (PK) deficiency with congenital haemolytic disease from 10 unrelated Italian families were characterized using the methods recommended by the International Committee for Standardization in Haematology (ICSH). All patients were double heterozygotes for the PK gene. The 10 variants were designated PK 'Lecce,' 'Parma,' 'Verona,' 'Milano,' 'Soresina,' 'Macerata,' 'Sassari,' 'Genova,' 'Mantova' and 'Brescia.' PK 'Sassari' was associated with glucose-6-phosphate dehydrogenase deficiency in two siblings. All mutants displayed multiple biochemical abnormalities except for PK 'Lecce' that only showed decreased red cell PK activity. No relation was found between the severity of anaemia and either the residual PK activity or specific biochemical enzyme abnormalities. Increased serum ferritin levels were detected in most of the patients, suggesting the need for systematically monitoring iron status in this disease.