OBJECTIVE Severe dengue affects all age groups in the Americas, but little detailed information is available about this disease in the elderly. The objective of this article is to describe the disease in this age group. METHODS We reviewed suspected dengue-case investigation forms submitted with diagnostic samples as well as clinical reports from infection control nurses in Puerto Rico, for the period of 1994 through 1999. RESULTS We assigned the laboratory-positive case-patients to four age groups: infants (1 year: 554), youth (2 to 18 years: 6 857), adults (19 to 64 years: 9 433), and elderly (>/= 65 years: 822). Regardless of infecting serotype, the elderly were more likely to have been hospitalized (48% vs. 33%) (P < 0.01) and were less likely to show hemorrhage (26% vs. 33%) (P < 0.01). On multivariate analysis, controlling for gender and the presence of hemorrhage, the elderly had a higher risk for hospitalization and death than did the youths and the adults. CONCLUSIONS The elderly appear to be more likely than youth and younger adults to develop severe illness when infected with the dengue virus, in a pattern similar to that of infants. The clinical evaluation of elderly patients with dengue must include a careful assessment of increased capillary permeability and occult hemorrhage in order to avoid complications from delayed identification and treatment of severe dengue infection. These findings are of increasing importance for dengue epidemiology and medical care in view of the expanding nature of dengue and dengue hemorrhagic fever in a world that also has a growing number and proportion of elderly persons.