Natural History, Genetics, and Pathophysiology of Systemic Juvenile Idiopathic Arthritis, Adult-Onset Still's Disease, and Related Conditions

* INCLUSION CRITERIA: Subjects with known or suspected sJIA, AOSD or a similar inflammatory phenotype will provide informed consent and then be evaluated either in the outpatient or inpatient unit of the NIH Clinical Center. To be eligible for follow-up visits patients must meet the Inclusion Criteria, but not the Exclusion Criteria. Subjects determined to not have known or suspected sJIA or AOSD, or a related inflammatory phenotype, will not be followed. Patients with signs and symptoms of sJIA will be classified as outlined in #1, #2 and #3 below: 1. Patients less than 16 years of age will be considered to have sJIA if they meet the ILAR criteria for sJIA. 2. Patients 16 years of age and older will be considered to have sJIA if they have previously met ILAR criteria for sJIA. 3. Family members of individuals included under items 1 and 2. 4. Controls for clinical, cellular, molecular, and biochemical assays, and genetic evaluation will be enrolled. Individuals who undergo phlebotomy specifically to provide a control specimen will include both pediatric and adult patients and will not be pregnant. Patients with signs and symptoms of AOSD will be classified as outlined in #1, #2 and #3 below: 1. Patients 16 years of age and older will be considered to have AOSD if they meet the Yamaguchi criteria for AOSD (including a negative ANA and RF). 2. Patients may be considered to have a diagnosis of AOSD if they met criteria for diagnosis in the past but do not still have present evidence of disease. 3. Family members of individuals included under items 1 and 2. 4. Controls for clinical, cellular, molecular, and biochemical assays, and genetic evaluation will be enrolled. Individuals who undergo phlebotomy specifically to provide a control specimen will include both pediatric and adult patients and will not be pregnant. Patients with suspected sJIA, AOSD or a related inflammatory condition, as indicated by the presence of episodic fever and/or arthritis, may also be included. EXCLUSION CRITERIA: 1. In adults, inability to provide informed consent and unavailability of a legally authorized representative to provide surrogate consent. In the case of minors, unavailability of a parent or guardian. 2. Presence of any medical condition that would, in the opinion of the investigators, confuse the interpretation of the study. 3. Unavailability, or inability to adhere with the schedule for follow-up visits. 4. Pregnancy