A subgroup of patients with haemophilia A who have a familial discrepancy between the one-stage and two-stage factor VIII:C results has previously been described. These patients show factor VIII:C levels by one-stage assay that are 2-7-fold higher than their two-stage results. We have studied 10 such families and identified six different mutations in the factor VIII gene in this group. The chemical cleavage method and DNA sequencing was used to identify mutations in factor VIII gene fragments generated by reverse transcription and PCR. All available family members were tested to confirm the presence of the mutation in affected individuals. These patients were found to have one of six single point substitutions causing a missense mutation and alteration to one codon in exons 7, 11, 14 or 18. The mutations comprise three that have not previously been described (Ala284Glu. Arg698Leu. Leu1932Phe) and three that have been previously described (Ser289Leu, Arg531His, Arg698Trp). Alterations to the amino acid composition of the A1, A2 and A3 domains of factor VIII are predicted by these molecular studies. In contrast, a control group of 23 mild haemophilia families with equivalent factor VIII:C results by one-stage and two-stage assays did not have any of the above mutations. Detailed studies in seven of these latter families identified four mutations affecting the A3, C1 and C2 domains of factor VIII. These findings suggest a genetic basis to the unusual factor VIII phenotype but do not explain the mechanism of the discrepant factor VIII activity.