The appearance of inhibitory antibodies against factor VIII (FVIII) is the most severe and costly complication of replacement therapy in patients with haemophilia A (HA). To determine the relationship between FVIII genotype and inhibitor development, baseline FVIII activity, genotype and inhibitor development were reviewed in 1104 patients with HA. In patients with severe HA, splicing errors present the highest frequency of inhibitors, ahead of inversion of intron 1 and of intron 22, nonsense mutations and large deletions. The lowest inhibitor frequency in severe HA is found in patients with missense mutations and small deletions/insertions. Subanalyses indicate that nonsense mutations and small deletions/insertions leading to a frameshift in the light chain are associated with a significant higher risk of inhibitor formation than similar mutations occurring in the heavy chain (27% vs. 14%). These mutation types also have a higher frequency of inhibitors when occurring in exons 23-26, where a second FVIII transcript originates, compared with similar mutations in exons 1-22 (28% vs. 17%). These results suggest that complete absence of FVIII because of null mutations, including splice site mutations, or the absence of a second transcript result in an increased risk of inhibitor development.