Koilonychia , also known as spoon nails, is a nail disease that can be a sign of hypochromic anemia, especially iron-deficiency anemia.

It refers to abnormally thin nails (usually of the hand) which have lost their convexity, becoming flat or even concave in shape. In a sense, koilonychia is the opposite of nail clubbing. In early stages nails may be brittle and chip or break easily.