The familial incidence of incontinentia pigmenti and its almost exclusive occurrence in females have been explained by two theories. One theory (Pfeiffer) suggests that the disease is due to an autosomal, dominant gene, which is sex-limited in its expression, producing the defect only in females. The other theory (Lenz) suggests that incontinentia pigmenti is due to a sex-linked gene carried on the X-chromosome. This gene acts as a dominant in females and is lethal in males. The pros and cons of each theory are discussed. The Franceschetti-Jadassohn type of incontinentia pigmenti is clinically and genetically different from the classic type. A third of "transitional" cases occurs in boys and girls. Its genetic mechanism has not yet been determined.