A partial gene deletion of about 1700 to 2000 bases spanning exon 3 and part of IVS-3 was identified in one patient with severe haemophilia A. Extensive DNA analysis of his family members revealed that the mother of the proband is a somatic mosaic for an abnormal factor VIII gene, because the defective gene could be identified in a considerable fraction of the leucocytes as well as of the cultured fibroblasts of the mother. This observation is important in estimating the recurrence risk in families of sporadic cases.