Twenty-nail dystrophy is a condition that affects the nails of the fingers and toes. The nails become rough, thin, and brittle. They appear similar to sandpaper. It was once thought that all 20 nails will be affected, but any number of nails can become rough and brittle. This condition most commonly occurs in childhood, but it can occur at any age. The exact cause is often unknown, but sometimes appears with other skin conditions such as lichen planus, psoriasis and alopecia areata. In some people, the symptoms of twenty-nail dystrophy improve or disappear over time, but in other people they persist for a long time.

The cause of twenty-nail dystrophy is unknown. In some cases, twenty-nail dystrophy has been diagnosed in people who also have another condition such as alopecia areata, or psoriasis. There are a few reported cases where several members of the same family have been affected by this condition, which suggests that there may be a genetic cause.

Although the symptoms of twenty-nail dystrophy usually begin in childhood, adults can also be affected by this condition. One article in the medical literature describes a woman who first showed symptoms of this condition at age 21. Another article describes an individual who was 63 when he first developed symptoms in his fingernails, but his toenails were not affected. He was diagnosed with tachyonychia, and twenty-nail dystrophy is thought to be a type of tachyonychia.

There is a recognized association between hair loss and twenty-nail dystrophy, but it is not known if one condition causes the other. Individuals with twenty-nail dystrophy often also have hair loss, though the extent of hair loss varies from mild to severe. In severe cases of hair loss, the individual may have a condition called alopecia areata.