G6PD deficiency is the most common enzymopathy in the world. The highest 
frequency values are found in tropical Africa, in the Middle East, in some areas 
of the Mediterranean, in tropical and sub-tropical Asia and in Oceania. This 
genetic defect shows sex linked inheritance and a marked heterogeneity. At least 
400 abnormal variants with different biochemical characteristics and about 100 
diverse mutations have been identified. In most cases the phenotypic expression 
is a marked decrease in erythrocyte G6PD activity. The most common clinical 
consequences are neonatal jaundice and sporadic haemolytic crises caused by a 
number of drugs, by infections or by ingestion of fava beans. A few cases of 
chronic non-spherocytic haemolytic anaemia associated with rare molecular 
variants have been reported. Early diagnosis, education and epidemiologic 
surveillance have been proved to be cornerstones in the prevention of the 
haemolytic disease. Therefore they should be taken into account in the national 
health programmes, especially in the countries with high prevalence rates.