A pill that appears to cure alopecia baldness has fully restored the hair of three patients.
Doctors conducted the pilot trial after identifying the immune cells responsible for destroying hair follicles.
Within four or five months of being put on the drug, ruxolitinib, all three patients experienced complete hair growth.
Dr Raphael Clynes from Columbia University Medical Centre in New York said: "If the drug continues to be successful and safe, it will have a dramatic positive impact on the lives of people with this disease."
Alopecia is an autoimmune disease that leads to partial or total hair loss. One of its best-known sufferers is former TV presenter Gail Porter (below).
There is no connection between alopecia and male pattern baldness that is hormone-driven.
Ruxolitinib is approved for the treatment of bone marrow cancer in the US and EU. Another drug, tofacitinib, is licensed for the treatment of rheumatoid arthritis in the US but not Europe.
The trial patients all had moderate to severe alopecia areata. Each was given a 20mg dose of ruxolitinib twice a day. The drug's effectiveness was linked to the disappearance of T-cell immune cells that attack hair follicles in the scalp.
The research appears in Nature Medicine journal.