The relationships in northern Thailand between fertility decline, AIDS-induced mortality and migration are analysed against a backcloth of important differences in modern demographic transformation between sub-Saharan Africa and Asia. For a peripheral agrarian society, northern Thailand has a remarkable combination of early, rapid fertility decline, extensive out-migration, and the highest incidence of AIDS in Thailand's substantial national epidemic. The interaction of these demographic changes, and their implications for development, are analysed through intensive survey work in four villages. The key concern for the future is found to be the aged-dependency ratio, and the uncertain ability to cope of the traditional, family-based system of support for the elderly.