A Thesis submitted to The University of Arizona College of Medicine - Phoenix in partial fulfillment of the requirements for the Degree of Doctor of Medicine. / The standard treatment for rattlesnake envenomation (RSE) is antivenom. The clinical course of patients treated with antivenom is well described. Prior to 2000, only a whole IgG AV (IgGAV) associated with high rates of hypersensitivity reactions (HSS) was available to treat RSE. Since 2000, Crotalidae Polyvalent Immune Fab (FabAV), which has a better safety profile than IgGAV, has been primarily used. Patients with RSE may not be treated with AV for a variety of reasons including history or perceived risk of HSS, patient refusal, drug shortage, or clinical impression that AV is not indicated. Research Question: What outcomes are associated with moderate to severe RSEs treated without antivenom?