The golden-mantled howler monkey (Alouatta palliata palliata) represents an excellent sentinel species for use in One Health modeling because of its extensive natural range, regular contact with humans and domestic animals, and susceptibility to zoonotic diseases. The purpose of this research is to establish baseline health assessments for free ranging golden-mantled howler monkeys in Marino Las Baulas National Park, Costa Rica. In April 2013, seven free ranging golden-mantled howler monkeys were anesthetized for physical examinations and biological sample collection, including blood for hematology and blood gas analysis. All seven howler monkeys studied appeared in good overall health. On physical exams, 4/7 (57%) had some level of dental disease, most notably crown fractures. Fecal samples for endoparasite evaluation were collected from the seven anesthetized animals as well as an additional three animals that were anesthetized but remained in the trees out of reach for full physical exams. Feces were collected from the rectum or collected from the ground immediately after defecation. Fecal analysis showed 9/10 (90%) had one or more species of intestinal parasites, and one animal tested positive for Giardia spp. Hematologic values showed a mild monocytosis and eosinophilia compared to other Alouatta spp. Trypanosoma spp. were present in the blood smear of one animal. Additional vector-borne disease testing found one individual PCR positive to Plasmodium spp., and one individual PCR positive to Venezuelan equine encephalitis virus. Antibodies for West Nile virus were found in 5/7 (71%) of the animals; however, PCR testing for West Nile virus and dengue virus was negative for all samples. This represents the first published health assessments for howler monkeys in this region.