During heavy seasonal rainfall in 1996, concurrent epidemics of dengue and leptospirosis occurred in an urban center in northeastern Brazil. We interviewed 110 cases of leptospirosis hospitalized a median of seven days after the onset of illness to evaluate the impact of the dengue epidemic on the triage of suspected leptospirosis from ambulatory clinics to the infectious disease reference hospital. Within the first three days of illness, 46 (42%) cases sought their first medical evaluation, and 28 (61% of 46) received a diagnosis of dengue. Dengue diagnoses were associated with a median of five days delay in referral to the infectious disease hospital. Patients who reported initial diagnoses of dengue were more likely than other patients to have required admission to the intensive care unit (odds ratio [OR] 2.7, 95% confidence interval [CI] 0.8–9.5) and to have died during hospitalization (OR 5.1, 95% CI 0.8–55.0). These findings indicate that diagnostic confusion between the early symptoms of leptospirosis and dengue may have contributed to the high mortality observed during the leptospirosis epidemic