From hospital records and referral by dermatologists 204 persons were ascertained through 75 probands who had inherited ichthyosis either as an autosomal dominant or as an X-linked recessive trait. Clinical examination of 148 of these patients confirmed previously recorded differences between the two varieties. Further features are described which may enable the sporadic case to be properly classified. Histological differences were also confirmed. In the X-linked families an effect of the gene was marked in one of the many heterozygous females. The Xg groups of the families finally established measurable distance between the locus for this X-linked blood group system and the locus for the X-linked form of ichthyosis, a linkage which had already seemed probable from studies of a series of families in Oxford, England, and Tel-Hashomer, Israel.