A 2-year-old boy with flat and spoon-shaped nail plates was referred to us. His mother had noticed that his fingernails were quite soft when he was born. After 2 months, the nails became flat or spoon-shaped; the toenail plates also became rough and flat. Systemic examination revealed nothing unusual. All his fingernail plates were quite thin, and had concave surfaces; the distal edges of some nail plates were rough and darkened (Fig. 1). His great, second and middle toenails on both sides showed the same abnormalities as the fingernails. Routine laboratory tests were normal; serum levels of iron, zinc, calcium, and magnesium were within normal limits. Repeated microscopic examination and culture of fungi from both fingernails and toenails were negative. Histologic examination of the distal edges of some of his fingernail plates was basically normal. When checking members of the family, we noticed that the boy's mother and maternal grandmother had similar nail problems (out of 14 members of three generations), although no systemic or other skin disorders were noticed. All the fingernails of the boy's mother were lusterless and concave with darkening of the distal edges (Fig. 2). All her toenails were rough, lusterless, and brown; some of them had longitudinal ridges. His maternal grandmother had light brown colored fingernail plates with concave profiles (Fig. 3), and dark brown or even blackish colored toenails. Both the mother and maternal grandmother acknowledged that their nails were abnormal from early childhood. Repeated microscopic examination and culture of fungi from both fingernails and toenails were negative. Several distal portions of the nail plates were collected for histologic examination. The fingernail plates of the boy's mother showed numerous parakeratotic cells in the middle zone, and those of his maternal grandmother showed diffuse parakeratotic cells; melanin granules were not found in those nail plates that were checked. Taken together, a diagnosis of familial koilonychia was established.