Incontinentia pigmenti is a rare genodermatosis typically involving three stages: vesiculae, verrucous lesions, and hyperpigmentation. We clinically and pathologically documented a case from shortly after birth until the age of 17 years. Although the first two stages took a regular course, the third stage of the disease was characterized by hypopigmented streaks on the legs in addition to axillary hyperpigmentation. Similar hypopigmented areas were found in the patient's mother. Because pathological investigations of hypopigmented areas have been extremely rare, we performed light and electron microscopic studies and compared these with our findings in hyperpigmented regions. Light microscopy showed the hypopigmented streaks with slight epidermal atrophy and a reduced number of melanocytes and skin appendages. However, the main finding was round eosinophilic bodies in the upper dermis. Electron microscopic examination of these bodies demonstrated amorphous material that resembled colloid, suggesting degeneration of basal keratinocytes. Confirming previous reports. in hyperpigmented areas we found a reduction of pigment in those parts of the basal layer overlying melanophages located in the upper dermis.