A 27-year-old male with hypomelanosis of Ito (HI) is reported. One of his two children had a postaxial ray defect of one leg but neither had cutaneous features of HI. Somatic mosaicism for a gene defect lethal to ectodermal derivatives offers the best explanation for HI in males, with consequent negligible recurrence risk. The limb defect is considered coincidental. The excess of girls with HI could be due to a female cohort with incontinentia pigmenti (IP) which may be indistinguishable: counselling of females must therefore take account of possible X-linked inheritance.