The GARP Study was established in 2000, aiming to identify genetic determinants of osteoarthritis (OA) susceptibility and progression. The study included 191 Caucasian sibling pairs of Dutch ancestry aged 43-79 years at baseline from multiple sites across the Netherlands. Recruitment occurred following the completion of screening questionnaires for symptoms and signs of OA, medical history and a family history of OA, and subsequent referral of siblings "with joint complaints" from index patients. All participants had OA at multiple joint sites in the hands or with OA in two or more of the following sites: hand, spine (cervical or lumbar), knee, or hip. Participants were also required to have symptomatic OA in at least one joint site, as defined following the American College of Rheumatology recommendations for knee and hip OA. A 2-year follow-up was conducted from 2002-2005, with participants evaluated again from 2007-2008 after an average of 5 years.