Abstract Incontinentia pigmenti is a familial disorder affecting tissues derived from neuroectoderm. Statistical analysis of reported pedigrees is consistent with transmission of incontinentia pigmenti by an X-Iinked dominant gene with male hemizygote lethality. This report describes a male infant with the classic clinical features of this condition and a 47.XXY chromosomal constitution. These findings support the concept that incontinentia pigmenti is an X-linked dominant disorder. This case illustrates the importance of a full genetic investigation in all males with physical findings suggestive of an X-linked dominant disorder lethal in males.