Beau lines are transverse depressions in the nail plate that are sometimes seen to be parallel to the shape of the lunula. Beau lines are common and nonspecific nail changes. They result from a sudden interruption of nail keratin synthesis and grow distally with the nail plate. They can vary with the width or depth of the depression, reflecting the duration or extent of the damage, respectively. As the nail plate grows distally, the Beau lines disappear.

Beau lines can be caused by trauma or local disease involving the nail fold, such as manicuring or paronychia, respectively. They may also result from metabolic or inflammatory influences, both local and systemic. When Beau lines are present in all nails at a similar location on the nail plate, there is likely to be a systemic cause, such as myocardial infarction, pemphigus vulgaris, Raynaud phenomenon, or rheumatic fever. As a drug effect, they are most commonly seen with chemotherapeutic agents. In immunosuppressed patients, they may be due to decreased nutrition from gastrointestinal disease or drugs.

In children, hand-foot-and-mouth-disease is a common cause of Beau lines.

Onychomadesis is a complete halt in nail plate production and is a severe form of Beau lines.