http://www.niams.nih.gov/Health_Info/Polymyalgia/default.asp
National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health
Polymyalgia rheumatica is a rheumatic disorder that is associated with moderate to severe muscle pain and stiffness in the neck, shoulder, and hip area. Stiffness is most noticeable in the morning. This disorder may develop rapidly--in some patients, overnight. In other people, polymyalgia rheumatica develops more gradually. The cause of polymyalgia rheumatica is not known; however, possibilities include immune system abnormalities and genetic factors. The fact that polymyalgia rheumatica is rare in people under the age of 50 suggests it may be linked to the aging process.