This study will examine the genetic and immune factors involved in the cause and development of psoriatic arthritis-a disease of both the skin and joints. It will describe the medical features and natural course of the disease and determine participants' eligibility for experimental treatment protocols.

Patients with known or suspected psoriatic arthritis 5 years of age and older and their relatives may enroll in this study. Patients will be evaluated with a medical history and physical examination, electrocardiogram, blood tests and X-rays. Additional procedures may include:

1. Leukapheresis-Collection of white blood cells for genetic analysis. 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 plasma is removed and the cells are returned to the body through a second needle placed in the other arm.
2. Skin biopsy-Removal of a small skin sample for microscopic analysis. An area of skin is numbed with an anesthetic and one to three small circular portions (about 1/4 inch in diameter) are cut and removed.
3. Joint aspiration-Removal of a small sample of synovial fluid (lubricating joint fluid). An area of skin around the biopsy site is numbed with an anesthetic, and a needle is inserted into the joint to pull out a small fluid sample.
4. Synovial needle biopsy-Removal of a small sample of synovial tissue (tissue lining the joint). An area of skin around the biopsy site is numbed with an anesthetic and a large needle is inserted into the joint. A smaller needle attached to a syringe is then placed inside the larger needle and small pieces of synovial tissue are removed.
5. Genetic studies-Saliva and blood samples will be collected for gene testing. Saliva is collected by rinsing the mouth with a tablespoon of salt water and spitting into a test tube.

Patients will be followed once or twice a year and may be evaluated for participation in an experimental treatment study.

Participating relatives of patients will fill out a brief medical history questionnaire and provide a DNA sample (blood sample or tissue swab from the inside of the cheek).