The aim of this project was to cross-culturally adapt and validate the American English version of the Childhood Health Assessment Questionnaire (CHAQ) and of the Child Health Questionnaire (CHQ) in the 32 different member countries of the Paediatric Rheumatology International Trials Organisation (PRINTO). This effort forms part of an international study supported by the European Union to evaluate the health-related quality of life in children with juvenile idiopathic arthritis (JIA) as compared to their healthy peers. A total of 6,644 subjects were enrolled from 32 countries: Argentina, Austria, Belgium, Brazil, Bulgaria, Chile, Croatia, the Czech Republic, Denmark, Finland, France, Georgia, Germany, Greece, Hungary, Israel, Italy, Korea, Latvia, Mexico, the Netherlands, Norway, Poland, Portugal, Russia, Slovakia, Spain, Sweden, Switzerland, Turkey, the United Kingdom, and Yugoslavia. A total of 3,235 patients had JIA (20% systemic onset, 33% polyarticular onset, 17% extended oligoarticular subtype, and 30% persistent oligoarticular subtype) while 3,409 were healthy children. This introductory paper describes the methodology used by all the participants. The results and the translated version of both the CHAQ and the CHQ for each country are fully reported in the following papers. The results of the present study show that cross-cultural adaptation is a valid process to obtain reliable instruments for the different socio-economic and socio-demographic conditions of the countries participating in the project.