Fibromyalgia syndrome (FMS) causes widespread muscle pain and tiredness, and is associated with anxiety and depression.
Sufferers are highly sensitive to touch, so that even a minor knock can be extremely painful.
The incurable condition, often confused with Chronic Fatigue Syndrome, or ME, is believed to be triggered by abnormally sensitive nerves.
Previous studies have shown that low levels of vitamin D are common in patients with FMS.
In the new study, 30 women with the syndrome who were lacking vitamin D were either given supplements of the vitamin or an inactive placebo.
Boosting vitamin D levels led to dramatic reductions in the treated group's experience of pain, as measured using a standard scale based on self-reported symptoms.