Factor VIII's exon 14 codes for its B domain that includes nearly one-third of its amino acid sequence that is not necessary for function. Frameshift mutations appear to occur more frequently within exon 14 than in other exons. To assess exon 14 frameshift mutations and their clinical correlates, a series of unrelated, severe or moderately severe haemophilia A patients were screened for heteroduplex formation in amplified exon 14 fragments. In 25 families, a frameshift mutation was identified by sequencing. Occurrence of haemophilia was isolated in 18 of these families. Moderate severity was noted in at least six out of 13 families with an A insertion or deletion at one of two sequences where the frameshift resulted in a sequence of 8-10 As. Inhibitors occurred in five of the other 12 families including one with an A insertion within a sequence of six As. Recurrent insertions into an A(8) (codons 1439-1441) or an A(9) (codons 1191-1194) sequence or of an A deletion from the A(9) sequence are common, recurrent causes of haemophilia A that may have a moderately severe phenotype.