Segmented heterochromia is a pigmentary disorder characterized by alternating dark and light segments on each hair shaft. Our aim was to observe the ultrastructure of hair shafts and bulbs to understand the pathogenesis of the disease. Biopsy specimens including normal or diseased hair follicles were prepared for transmission electron microscopy. In dark segments, dense and ellipsoidal melanosomes were noted while small, round ones were found in the light segments. Two groups of melanosomes lined up on the matrix, one of which was composed of compact ellipsoidal melanosomes, and the other was composed of melanosomes with irregular sizes and shapes. Melanocytes seemed to be under necrosis, apoptosis, or dark cell transformation. Langerhans cells were found in the bulb. Two kinds of melanosomes were produced in the same hair bulb simultaneously. Degenerating melanocytes might produce deformed melanosomes. Langerhans cells might be involved in the death of melanocytes. It is unknown how either group of melanosomes is adopted and transferred to the hair cortex producing the characteristic pattern of pigmentation.