Objectives : The expression density of GPIaIIa, the primary platelet collagen receptor (integrin alpha2beta1), is linked to two polymorphisms (GPIa-807C/T and HPA-5a/b). During evolution a gene shift from the genotypes GPIa-807CC-HPA-5bb to the genotypes GPIa-807CT-HPA-5aa has taken place. The aim of the study was to assess whether iron deficiency anaemia (e.g. increased blood loss) in young women could be associated with a specific genotype, indicating a role as potential evolutionary selection factor. Study design : Women between 18 and 40 yr of age presenting for their first blood donation were asked about alimentary habits and use of oral contraception. Haemoglobin and serum ferritin were measured and the GPIa-C807T and HPA-5 genotypes were determined. Results : Two hundred women were included and grouped according to the WHO definition for iron deficiency anaemia (haemoglobin <121 g L-1 and ferritin <15 mug L-1). Eight women fulfilled both WHO-criteria for iron deficiency anaemia, 145 women fulfilled none. No differences regarding age, use of oral contraceptives, alimentary habits, and HPA-5 were found between the groups. The GPIa-807CC genotype was strongly over-represented in the WHO-anaemic women as compared to the non-WHO-anaemic women (87.5% vs. 33.1%, P=0.003). Conclusion: Iron deficiency anaemia in young women might have been the evolutionary disadvantage causing the gene shift from GPIa-807CC to 807CT.