[Show abstract][Hide abstract] ABSTRACT: TO THE EDITOR: Leptospirosis, caused by Leptospira spp. bacteria, and dengue, caused by dengue viruses (DENVs), are potentially fatal acute febrile illnesses (AFI) endemic to the tropics (1,2). Because their clinical manifestations are similar (3), leptospirosis may be misidentified as dengue (4). We report a fatal case of co-infection with Leptospira spp. and DENV-1 in a man in Puerto Rico.