The proportion to which α-thalassaemia contributes to anaemia in Africa is not well recognized. In an area of intense malaria transmission in South-West Nigeria, haematological parameters of α-thalassaemia were examined in 494 children and 119 adults. The −α3.7 type of α+-thalassaemia was observed at a gene frequency of 0.27. Nine and 36.5% of individuals were homozygous and heterozygous, respectively. P. falciparum-infection was present in 78% of children and in 39% of adults. The α-globin genotypes did not correlate with the prevalence of P. falciparum-infection. α+-thalassaemic individuals had significantly lower mean values of haemoglobin, mean corpuscular volume, and mean corpuscular haemoglobin than non-thalassaemic subjects. Anaemia was seen in 54.7% of children with a normal α-globin genotype, in 69.9% of heterozygous (odds ratio: 1.99, 95% confidence interval: 1.32-3.00, P = 0.001), and in 88.4% of homozygous α+-thalassaemic children (odds ratio: 7.72, 95% confidence interval: 2.85-20.90, P = 0.0001). The findings show that α+-thalassaemia contributes essentially to mild anaemia, microcytosis, and hypochromia in Nigeria.