Eighteen male patients with incontinentia pigmenti (IP) showed the characteristic clinical features and, when examined, histologic skin defects observed in female patients with IP. Six of the patients had neurologic, ophthalmologic, or dental manifestations as well. Three patients showed evidence by polymerase chain reaction analysis of both the normal NEMO gene and the exon 4-10 deletion in NEMO that occurs in the majority of affected girls with IP, confirming postzygotic mosaicism for the NEMO gene.