Fetal hemoglobin (HbF) levels determined in healthy Japanese adults ranged from 0.3% to 16.0% as F cells and 0.17% to 2.28% as HbF content, which were the same as those obtained in other countries. The frequency distribution of 300 healthy adults with various numbers of F cells consisted statistically of two different groups, low and high F-cell groups. Individuals with greater than or equal to 4.4% of F cells (HbF about 0.7%) were defined as the high F-cell trait, which accounted for 11.3% of males and 20.7% of females. Family studies of 21 probands with this trait and sex-different frequency analyses in the population and probands revealed X-linked dominant inheritance. Two other families of the trait associated with color blindness were described, although no definitive evidence for linkage was obtained between the two. A review of population and family studies reported in the literature indicated that persons with Swiss-type hereditary persistence of fetal hemoglobin (HPFH) are of the same kind as this trait in their incidence and inheritance form, but represent a portion of the trait with higher levels of HbF or F cells. The existence of X chromosome-localized regulatory gene(s) for the developmental switch of human Hb production is discussed.