Some boys with X-linked chronic granulomatous disease (CGD) have red cells of the rare McLeod phenotype in the Kell blood group system. Only one example of this phenotype has previously been described in a non-CGD subject. We have studied a 10-year-old boy and a maternal brother who do not have CGD and whose red cells are of the McLeod type . The boy presented as a haematological problem with red-cell abnormalities. These were acanthocytosis, anisocytosis and 'tailing' in the osmotic fragility curve, changes now known to occur with the McLeod phenotype. Subsequent studies revealed his rare blood group. A family study has established that an uncle also has acanthocytic red cells and the McLeod phenotype. In addition the boy's sister, mother and maternal grandmother all show red-cell mosaicism with double populations of McLeod acanthocytes and normal red cells of common Kell type. The gene that determines inheritance of the McLeod phenotype is X-linked and the mosaicism present in female carriers is believed to result from X chromosome inactivation by the Lyon effect. The study provides further evidence that the McLeod phenotype arises by inheritance of a variant X-linked modifying gene and not through inheritance of a variant gene at the Kell autosomal locus. It also represents the first occasion that a person of rare blood group has been recognized because of an associated anomaly in red cell morphology.