This study will evaluate subjects with adult- and childhood-onset myositis to learn more about their cause and the immune system changes and medical problems associated with them. Myositis is an inflammatory muscle disease that can damage muscles and other organs, resulting in significant disability.

Children or adults with polymyositis or dermatomyositis or a related condition may be evaluated under this study. Healthy children or adults will also be enrolled as "controls," for comparison of test results.

All patients will undergo a complete history (including completing some questionnaires) and physical examination, review of medical records, and blood and urine tests. Patients may then choose to participate in an additional 1- to 5-day evaluation, which will include some or all of the following diagnostic, treatment or research procedures:

1. Standardized muscle strength testing, range of motion of joints and walking (gait) analysis by a physiotherapist; completion of a questionnaire regarding ability to perform daily tasks
2. Skin assessment, possibly including photographs of lesions and a skin biopsy (removal of a small skin sample under local anesthetic)
3. Magnetic resonance imaging (scans that use magnetic fields to visualize tissues) of leg muscles
4. Swallowing studies, including a physical examination and questionnaire on swallowing ability, studies of tongue strength, and ultrasound imaging during swallowing, and possibly, a modified barium swallow
5. Voice and speech assessment, possibly including computerized voice analysis and laryngoscopy-analysis of the larynx (voice box) using a small rigid scope with a camera placed in the mouth to view and record vocal cord function
6. Pulmonary function tests (measurement of air moved into and out of the lungs, using a breathing machine) to evaluate lung function and, possibly, chest X-ray
7. Electrocardiogram (measurement of the electrical activity of the heart) and, possibly, echocardiogram (ultrasound imaging of the heart)
8. Endocrine evaluation
9. Eye examination, in patients with vision loss or other eye symptoms
10. Nutrition assessment to evaluate muscle mass and muscle wasting, including tape measurements or bioelectric impedance testing, a painless procedure in which wires are attached to the extremities with a sticky paste.
11. Muscle ultrasound.
12. Electromyography (record of the electrical activity of muscles)
13. Muscle or skin biopsy (removal of a small piece of muscle tissue for microscopic examination)

All patients may have only a one-time evaluation or may return for one follow-up evaluations (either the 1-day or 3- to 5-day evaluation) over a 1-year period.

Healthy children will undergo a medical history and brief physical examination; blood and urine tests; speech and swallowing studies including questionnaires and physical examination, tongue strength, and ultrasound study; and bioelectric impedance testing. Children 8 to 18 years old may also have exercise testing.