The Cohort Hip and Cohort Knee (CHECK) study included participants with early symptomatic osteoarthritis (OA) of the hip or knee and evaluated clinical, radiographic and biochemical variables in order to establish the course, prognosis and underlying mechanisms of early symptomatic osteoarthritis. A total of 1002 participants aged 45-65 years, with symptomatic OA characterized by pain of knee and/ or hip, entered the cohort in the period October 2002 to September 2005. They were included at or within 6 months of their first visit to the general practitioner for these symptoms. An overview of measures that are included in the study can be found on the website [www.check-research.com]. On the basis of their presenting symptoms, participants were divided into two groups. Participants with mild symptoms visited the research centre at years 0, 2, 5, 8 and 10 (variable visiting group) and participants with more serious symptoms visited the research centre each year (annual visiting group). After 7 years, only 105 participants (10%) had dropped out; their baseline characteristics did not differ significantly from those of other participants. CHECK is a valuable source of information on early symptomatic OA, that allows the examination of high-quality data on clinical, radiographic and biochemical variables. The CHECK steering group welcomes collaboration with national and international colleagues. Requests for collaboration or access to data can be sent to [checkreu@umcutrecht.nl].