the aim of the study reported here was to demonstrate that dengue hemorrhagic fever occurs in puerto rico, that it is underreported, and that this underreporting is due partly to underdiagnosis in hospitals. surveillance for severe dengue identified 986 hospitalizations for suspected dengue in 1990-1991. at the time, on the basis of available clinical and laboratory data, the surveillance system routinely identified 20 dhf cases, including three with dengue shock syndrome (dss). our subsequent review of these 986 patients' hospital records identified 102 whose records supported a clinical diagnosis of dhf (88) or dss (14). of the 102, there were 57 with positive virologic or serologic results for dengue and that met the world health organization criteria for dhf (fever, hemorrhagic manifestations, thrombocytopenia, and excessive capillary permeability). this group of 57 patients had a mean age of 38 years, contained a preponderance of males (34, 59.3%), included eight cases of dss, and involved two (3.5%) fatalities (in females 16 and 55 years old). hemorrhagic manifestations were mild; hemoconcentration, hypoalbuminemia, and elevated aspartate and alanine aminotransferase (ast and alt) levels were frequently encountered. the median duration of hospitalization was five days. the clinical description of these laboratory-positive dhf cases in puerto rico is consistent with previous descriptions of dhf in the medical literature; but the patients' age distribution is similar to the pattern typically found in the americas (where all age groups tend to be affected), as opposed to southeast asia (where mostly younger children are affected). the number of dhf cases identified by our study was nearly three times that reported through the established surveillance system. our findings indicate that recognition and reporting of dhf by local clinicians needs to be improved.