Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most commonly known inherited disorder in man, and is estimated to affect 400 million people worldwide.' The highest prevalence rates (with gene frequencies in the range of 5-25% are found in tropical Africa, the Middle East, tropical and sub-tropical Asia, some parts of the Mediterranean, and in Papua New Guinea. The spectacular advances in molecular biology of the last decade are helping to unravel the molecular basis of its biochemical and clinical diversity. It serves as a model for the clinician of the importance of the environment in determining the clinical expression of genetic disease. For the population biologist, its study has yielded important insights into the interaction of host red cells with the malaria parasite and the influence of this interaction on human genetic polymorphism. Most human pathology due to G6PD deficiency is preventable by population screening and avoidance of precipitants, thus posing a challenge for epidemiologists and community physicians. There are several excellent recent reviews.2-5