Background and Objectives: Our aim was to construct a harmonized measure of activities of daily living (ADL) across six countries, and to evaluate the reliability and validity of this measure. Methods: A population of 9,297 persons, aged 65-89 years, was drawn from the Comparison of Longitudinal European Studies on Aging (CLESA) study, which includes data from five European countries and Israel. Because the number, type, and response format of the ADL items differed across the six studies, a four-item scale was constructed to harmonize the data, using items common to most countries. A procedure was devised to substitute or construct items that were not available in two of the countries. Results: Cronbach's α for the four-item ADL measure varied from 0.81 in Spain to 0.92 in Finland, and was similar to the α of scales including five or six items. Kappa scores between substituted or constructed items and the actual items varied from 0.50 to 0.78. In all countries, the percentage of persons with ADL disability differed significantly across age and was associated with chronic diseases, poor self-rated health, global disability, and home help utilization. Conclusion: The harmonized four-item ADL measure seems a reliable and valid instrument for comparing ADL disability in older people across countries.