Epidemiologic studies of fibromyalgia have so far been based on rheumatologic and general practice settings, which are poor proxies for the underlying population. The study is based on a national health interview survey carried out by the Danish Institute for Clinical Epidemiology in 1990/91 on approx. 6000 randomly selected Danish citizens. For this study 1219 subjects from the eastern part of Denmark aged 18 to 79 years were asked about widespread muscle pain. One-hundred-and-twenty-three persons fulfilled the screening criteria. Clinical examination could be performed on 65 persons (53%). Eight subjects, all female, met the 1990 American College of Rheumatism criteria for fibromyalgia. Dropouts were regarded as not having fibromyalgia. The prevalence of fibromyalgia in the Danish population between 18 and 79 years of age was found to be a minimum estimate of 0.66% (95% confidence limits 0.28%-1.29%).