Some mosaic conditions may affect the haemophilia phenotype. Well-known instances include chromosomal mosaicism because of aneuploidy and pseudo-mosaicism because of varying patterns of X-chromosome inactivation. Chromosomal mosaicism in a chimera is a potential source of phenotypic variation. Gene mosaicism is commonplace. Its pattern and effect depend on the stage of development at which a mutation occurs. Proven or possible genetic mosaicism is an important consideration when predicting the likelihood of transmission of haemophilia to a future generation. A mosaic is an individual who has two or more cell lines, genetically different with regard to chromosomes or genes. As techniques improve and studies accumulate, mosaics are being found to be more common than hitherto believed. Some mosaic conditions may affect the phenotype of haemophilia in males and of the carrier state in females. Cells may be mosaic with regard to chromosomes, as in some instances of aneuploidy, and in chimeras, and in females owing to the pattern of X-chromosome inactivation. Cells may be mosaic with regard to new gene mutations. The pattern of mosaicism depends upon the stage in embryogenesis or in germ-cell formation in which the mutation arose.