Dengue fever is probably the most important arthropod-borne viral infection worldwide. The World Health Organization estimates an incidence of 100 million cases per year. Dengue fever is of clinical importance mainly in tropical and subtropical zones, but the increasing number of travellers visiting these endemic areas means that physicians in temperate zones will meet this tropical infectious disease more frequently. To estimate the frequency of dengue fever in Switzerland, we retrospectively analyzed a cohort of patients in whom clinicians wished to rule out a case of malaria on the assumption that some of these patients may have been exposed to dengue virus while travelling in tropical and subtropical areas. Dengue was diagnosed by serology on contemporary stored serum samples with a commercially available assay that has been evaluated before. During the study period (1.1.93 to 1.7.94) a malaria diagnosis was requested by the physicians in our center in 87 cases and malaria was diagnosed in 10 (11.5%). In 38 of these 87 cases a stored serum sample was available for dengue serology. All of these patients had visited an area where not only malaria but dengue virus was endemic. In 7 of these 38 cases (18%) malaria was diagnosed. In 3 patients (8%) the dengue serology (IgM and IgG) was indicative of acute dengue fever. All 3 cases had clinical findings compatible with the disease. In 4 more patients (10%) anti-dengue IgG antibodies were found. All of these 4 patients had previous exposure to dengue virus but no acute clinical illness.(ABSTRACT TRUNCATED AT 250 WORDS)