Alopecia areata, also known as spot baldness, is a condition in which hair is lost from some or all areas of the body. Often, it results in a few bald spots on the scalp, each about the size of a coin. Psychological stress and illness are possible factors in bringing on alopecia areata in individuals at risk, but in most cases there is no obvious trigger. People are generally otherwise healthy. In a few cases, all the hair on the scalp is lost (alopecia totalis), or all body hair is lost (alopecia universalis). Hair loss can be permanent, or temporary. It is distinct from pattern hair loss, which is common among males.

Alopecia areata is believed to be an autoimmune disease resulting from a breach in the immune privilege of the hair follicles. Risk factors include a family history of the condition. Among identical twins, if one is affected, the other has about a 50% chance of also being affected. The underlying mechanism involves failure by the body to recognize its own cells, with subsequent immune-mediated destruction of the hair follicle.

No cure for the condition is known. Some treatments, particularly triamcinolone injections and 5% minoxidil topical creams, are effective in speeding hair regrowth. Sunscreen, head coverings to protect from cold and sun, and glasses, if the eyelashes are missing, are also recommended. In more than 50% of cases of sudden-onset localized "patchy" disease, hair regrows within a year. In patients with only one or two patches, this one-year recovery will occur in up to 80%. However, most patients will have more than one episode over the course of a lifetime. In many patients, hair loss and regrowth occurs simultaneously over the course of several years. Among those in whom all body hair is lost, fewer than 10% recover.

About 0.15% of people are affected at any one time, and 2% of people are affected at some point in time. Onset is usually in childhood. Females are affected at higher rates than males.