An outbreak of dengue fever occurred in Taiwan between 1987 and 1988. The highest attack rate among adults was estimated at 5.6% in the city of Kao-hsiung. A case-control study was carried out to determine the risks of contracting dengue infection and to identify protective factors against the infection. One hundred dengue patients of the authors' hospital who were diagnosed by virologic or serologic tests constituted the case group. Each dengue patient was matched to a control patient of the same age and sex who had been diagnosed as suffering from a non-vector-borne disease on the same day as the dengue patient. Of the household protective measures against dengue infection prior to the occurrence of illness, the adjusted odds ratio, estimated by stratified analysis, was lower for people who lived in screened houses (odds ratio = 0.58, 95% confidence interval 0.36-0.92) as compared with inhabitants of unscreened houses. The odds ratio was as low as 0.18 (95% confidence interval 0.06-0.56) for people whose homes were fully screened with door screens opening outwardly. Patients who lived near markets and/or open sewers or ditches were running a risk of dengue infection 1.8 (95% confidence interval 1.3-2.4) times higher than those who lived elsewhere. To control dengue outbreaks, the authors recommend that special attention should be devoted to the reduction of outdoor vector sources. Full screening, especially outwardly opening screen doors, seems to be an individual's best protection against dengue fever.