We report the analysis by single-stranded conformation polymorphism of the essential sequences of the factor VIII (FVIII) gene (total length about 14 kb) including the entire coding sequence, flanking intronic sequences and the putative regulatory sequences 5' to the gene, in twelve unselected haemophilia A patients of Portuguese origin. Direct sequencing of the fragments with an altered migration pattern led to the identification of the disease-producing mutations in five patients. Three of these mutations, namely a 1 bp insertion in a motif of eight consecutive A residues at codon 1439 (FVIIIPorto3); a C to T transition at codon 1966 (Arg→Stop), found in an inhibitor-positive patient (FVIIIMontijo); and a G to A transition at codon 479 (Gly→ Arg; FVIIIPorto1), have been reported in other ethnic groups. The two novel mutations are the substitution of AG by GG at the 3' end of intron 4 (FVIIILisboa1 destroying the invariant splice acceptor sequence, and a G to A transition at codon 1948 resulting in an aspartic acid substitution for glycine (FVIIIporot2).