A panel of 27 families at risk for haemophilia A was studied by RFLP analysis using the anonymous probe St14.1 (DXS52), a cDNA probe spanning the exons 16 to 19, and a genomic fragment of intron 22. In two patients with severe haemophilia A, who did not form inhibitors, abnormal RFLP patterns were found, that can be interpreted as partial deletions in exons 17 to 19, and intron 22, respectively. In a case with moderate haemophilia A a further partial deletion in intron 22 was found. The significance of the deletions detected as markers for pedigree analysis and prevention of haemophilia A is demonstrated.