Haemophilia A is caused by a broad range of mutations in the factor VIII (FVIII) gene. The most frequent of them is a large inversion, which appears to be the underlying defect in approximately 45% of all severely affected patients (FVIII < or = 1%). The results here are of 84 unrelated Slovak haemophilia A cases. The factor VIII inversion was identified in 22 of 44 (50%) patients with severe haemophilia A and in 1 of 13 patients with moderately severe disease. The inversions of distal type were more frequent (82.6%) than proximal ones (17.4%).