Capture-recapture estimations compare the results of 2 or more independent surveillance systems for the same event, and by measuring the degree of overlap between them, provide an estimate of the total number of events, and therefore the completeness of ascertainment in each system. The Puerto Rico Department of Health and the Dengue Branch of the Centers for Disease Control and Prevention (CDC) monitor dengue activity in Puerto Rico through 2 distinct surveillance systems: diagnostic specimens from patients with suspected dengue and infection control nurses' reports on patients hospitalized for suspected dengue. The patient listings from these systems were used in a 2-sample, capture-recapture calculation to estimate the total number of persons with suspected dengue hospitalized from 1991 to 1995. The laboratory positivity rate for suspected dengue cases who submitted appropriately timed serum samples in those years ranged from 72.1% to 81.2%. The laboratory-based (diagnostic sample) surveillance system (routinely used to monitor hospitalizations for suspected dengue) detected an average of 1,197 hospitalized cases during non-epidemic years, and 4,329 cases during the epidemic year of 1994. The detection rate of this system averaged 42% of the numbers derived by the capture-recapture method. In non-epidemic years, an estimated average of 2,791 patients (range = 1,553-3,481) was estimated to have been hospitalized with a clinical diagnosis of dengue, compared with 9,479 during 1994. These results demonstrate the under-detection inherent in passive surveillance systems for hospitalized cases of suspected dengue, and illustrate the value of capture-recapture techniques to better estimate the true incidence of hospitalizations for this disease.