Epidemiological research in the field of paediatric rheumatology is important for reasons such as the identification of possible aetiological factors, description of the natural history, identification of biologically homogeneous disease groups and health care planning. This review will focus on the epidemiology of 'idiopathic' juvenile arthritis (JA) where data are available for comparison in terms of time, space and ethnic origin. Methodological issues that make comparisons of data difficult, such as case definition, source population and case ascertainment, will be discussed in relation to the data presented. The incidence and prevalence of JA cover a wide range, but, in studies that use similar methodology, an incidence of 5-18 and a prevalence of 30-150 per 100,000 children under the age of 16 is found in Europe and on the American continent. Studies from other parts of the world indicate differences in age of onset and subgroup distribution related to geographical location and/or race.