Seventeen families of patients with primary fibromyalgia were studied for evidence of inherited primary fibromyalgia. Fifty parents and siblings were included in the analysis. Twenty-six (52%, mean age 33.5 years) had characteristic symptoms and findings of primary fibromyalgia. Eleven (22%, mean age 28 years) were asymptomatic but had clinical evidence of abnormal muscle consistency to palpation without tender or trigger points. One person had clinical evidence of lupus. Thirteen (26%) had no evidence of fibromyalgia or abnormal muscle consistency. The mode of inheritance was autosomal dominant. Identical twins are described who developed symptoms of primary fibromyalgia within six months of each other, as are two brothers who developed abnormal palpable muscle consistency years before acquiring the characteristic findings of the fibromyalgia syndrome. Primary fibromyalgia may be an inherited condition with a variable latent stage before clinical expression of the disease.