Purpose: To develop a Thai questionnaire ชีวิตและสุขภาพของวัยรุ่นในประเทศไทย (TYQ) to explore girls’ and boys’ living conditions, lifestyles, and self-reported health with special focus on sexuality, based on a Swedish questionnaire, Liv & Hälsa ung (SYQ). Challenges in developing a youth questionnaire for comparative studies are described. Design: A multistep translation, sociocultural adaptation procedure, and a mixed-method validation test were performed using English as a common language within the research group. Three versions of SYQ were used as a pool of questions to develop the questionnaire. Findings: From a field test, unclear questions were identified and minor adjustments made. Life & Health Young People in a Thai version was successfully developed. The English version was used to bridge the language gap. Conclusion: This unique multistep methodology, including mixed-method validation procedure, can be used by researchers in countries where English is not the main language. [ABSTRACT FROM AUTHOR]