Aims: The present paper focuses on the measurement of health literacy (HL), which is an important determinant of health and health behaviours. HL starts to develop in childhood and adolescence; hence there is a need for instruments to monitor HL among younger age groups. These instruments are still rare. The aim of the project reported here was therefore to develop a brief, multidimensional, theorybased instrument to measure subjective HL among school-aged children. Methods: The development of the instrument covered four phases: item generation based on a conceptual framework, a pilot study (n=405), test-retest (n=117), and construction of the instrument (n=3853). All the samples were taken from Finnish 7 th and 9 th graders. Results: Initially, 65 items were generated, of which 32 items were selected for the pilot study. After item reduction, the instrument contained 16 items. The test-retest phase produced estimates of stability. In the final phase a 10-item instrument was constructed, referred to as Health Literacy for School-aged Children (HLSAC). The instrument exhibited a high Cronbach alpha (.93), and included two items from each of five predetermined theoretical components (theoretical knowledge, practical knowledge, critical thinking, self-awareness, citizenship). Conclusions: The iterative and validity driven development process made it possible to construct a brief multidimensional HLSAC instrument. Such instruments are suitable for large-scale studies, and for use with children and adolescents. Validation will require further testing for use in other countries.