White macules are present in more than 50% of the patients with tuberous sclerosis when no other visible lesions typical of tuberous sclerosis have appeared; the majority, if not all, of the white macules appear at birth. In fair-skinned infants the lesions may be easily overlooked and may only be detected with Wood's light. The configuration of the white macules may be oval or linear; the most characteristic outline, however, is the lance-ovate shape. Cytologically altered melanocytes and decreased tyrosinase activity and reduced melanin deposition on melanosomes are present in the hypomelanotic macules of tuberous sclerosis, whereas melanocytes are very rarely present in the hypomelanotic macules of vitiligo. Therefore, the cytological changes of the melanocyte system in the white macules of tuberous sclerosis are different from those present in the white macules of vitiligo.