Most communicable diseases in Singapore have been brought under control and some eliminated. In recent years, an increasing proportion of the reported cases turned out to be imported. Between the period 1977 and 1982, 96% of malaria, 44% of paratyphoid, 32% of typhoid, 20% of leprosy, 11% of acute viral hepatitis, 7% of dengue fever/dengue haemorrhagic fever and 7% of cholera were imported. About 10% of the notified tuberculosis cases were non-residents while all the sporadic cases of poliomyelitis (except in 1977) and diphtheria (except in 1982) were contracted outside Singapore. The majority of the infections originated from Southeast Asia and the Indian subcontinent. The main groups of population with imported infections were local residents who travelled to the endemic areas without taking adequate preventive measures, foreign contract workers, and foreign seeking medical treatment in Singapore. Whether or not these imported cases would spread the infection to others in the community and cause epidemics depend on the virulence of the pathogen introduced, the susceptibility of the population and the environmental conditions which favour transmission of infection. Measures taken to reduce the risk of transmission include provision of a high standard of environmental sanitation, epidemiological surveillance to detect and eliminate the focus of infection; maintenance of a high level of herd immunity through immunisation; health education of the medical practitioners and of the public on the need for personal prophylaxis when travelling overseas; and screening of foreign contract workers and returning residents in special situations.