Epidemiological studies of chronic arthritis in childhood can provide clues to genetic determinants of disease manifestations and environmental triggers. Available data are difficult to compare, however, because of the heterogeneity of the disease, differences in the classification criteria used for definition and inclusion, and differences in source populations and case ascertainment. Nevertheless, when the data are interpreted according to the methodologies used, geographical and ethnic differences can be found with regard to occurrence rates, age at onset, subgroup distribution and immunological markers. Seasonal variations have been detected in systemic disease. Variations in the incidence of childhood arthritis over time have also been observed, indicating environmental influences on disease frequency, while familial aggregations suggest the presence of genetic factors. These epidemiological data from a challenging puzzle which we hope will provide clues to future understanding of etiologies and cures, with the help of basic scientific research.