This study of inflammatory muscle diseases-polymyositis and dermatomyositis and related disorders-will examine what causes these diseases and describe the clinical features (signs and symptoms) associated with them. Inflammation and degeneration of skeletal muscles in these disorders leads to weakness and muscle wasting. The skin, lungs and other organs may also be involved.

Patients 16 years of age and older with polymyositis, dermatomyositis, or a related disorder may be eligible for this study. Participants will undergo a complete history and physical examination, including routine blood and urine tests. Additional procedures for diagnosis, treatment or research may include:

1. Blood sample for genetic studies.
2. Muscle biopsy-removal of a tissue sample for microscopic examination. Under local anesthetic, a 1/2- to 1-inch long incision is made in the thigh or upper arm, and a small piece of muscle is removed.
3. Electromyography-measurement of the electrical activity of a muscle. A needle is inserted through the skin into a muscle to record its electrical activity.
4. Magnetic resonance imaging-visualization of organs or tissues, using a magnetic field and radio waves. The patient lies on a table inside a narrow cylinder (the MRI scanner) with a strong magnetic field for the scanning.
5. Manual muscle strength testing by a physiotherapist.
6. Swallowing studies using ultrasound (imaging using sound waves) and X-rays (barium swallow) to evaluate swallowing and speaking abilities.
7. Questionnaires on swallowing ability and ability to perform daily living activities
8. Pulmonary function tests-measurement of movement of air in and out of the lungs. The patient breathes into a machine to evaluate lung function.
9. Chest X-rays to evaluate lung function.
10. Electrocardiogram and, if necessary, Holter monitoring (measurement of the electrical activity of the heart) and echocardiogram (ultrasound imaging of the heart) to evaluate heart function.
11. Apheresis-collection of white blood cells for research. Whole blood is collected through a needle placed in an arm vein. The blood circulates through a machine that separates it into its components. The white cells are removed and the rest of the blood is returned to the body through the same needle or through a second one placed in the other arm.
12. MR guided muscle biopsy-measurement of glycogen in muscle tissue using magnetic resonance imaging. Certain patients may undergo this experimental procedure to compare MRI findings with those of muscle biopsy. The affected muscles are identified using MRI and the biopsy incision is made. MRI is then used to guide the biopsy needle to the muscle and a small piece is removed.

Patients who are eligible for experimental treatment studies will be offered the opportunity to join them. Others will be advised of treatment recommendations.