Vector-borne virus infections were studied in 670 German overseas aid workers who had spent an average of 37.7 months in tropical areas of Africa and Asia. Antibodies to dengue viruses (DEN) were detected by indirect immunofluorescence assay in 43/670 (6.4%) aid workers. Of these 43, 41 (95.3%) were also positive for antibodies to dengue by haemagglutination inhibition assay. The highest seroprevalence was in aid workers returning from Thailand (19.4%), Benin (14.8%) and Burkina Faso (9.2%). Antibodies to chikungunya virus (CHIK) were detected in 9/670 (1.3%) aid workers, and the highest seroprevalence to anti-CHIK IgG was in aid workers who had resided in Benin (5.7%) and Thailand (5.5%). Antibodies to Sindbis virus were detected only in 1/670 (0.1%) aid worker who had been to Zambia. Vector-borne virus infections, especially DEN, pose a health risk for aid workers.