Bloch-Sulzberger incontinentia pigmenti (IP) is a rare X-linked neuroectodermal syndrome. Over 97% of the patients are female. We report on a male baby who developed blisters in linear groups or bands shortly after birth. When the child was 3 months old the blisters were followed by verrucous papules, which cleared after 1 year leaving areas of brownish grey hyperpigmentation. In addition to the skin involvement, our patient showed central motor dysfunction on the right side of the body and also dental and ocular anomalies. Both parents were in good health. Chromosome analysis yielded a normal karyotype (46, XY). The genes for coagulation factor VIII and biglycan in the Xq28 region were not deleted. The presence of the disease in this male infant may be due to an early somatic mutation or a half-chromatid mutation. A further possibility is mosaic expression of an unstable premutation. This model offers a good explanation for the reports in the literature of transmission of the disease from mother to son.