Avian trichomonosis, a disease typically caused by the protozoan parasite Trichomonas gallinae, is a well recognized cause of death in many avian species. In California, US, trichomonosis has caused periodic epidemics in Pacific Coast Band-tailed Pigeons (Patagioenas fasciata monilis). We summarize reported mortality events and investigate ecologic drivers and population impacts associated with epidemic mortality due to trichomonosis in Band-tailed Pigeons. Between 1945 and 2014, 59 mortality events involving Band-tailed Pigeons were reported in California with the number of reported events increasing over time. Estimated mortality for these events was variable, ranging between 10 and 10,000 pigeons. Events were most-frequently reported in Monterey (19%; 11/59) and San Luis Obispo (8%; 5/59) counties. Events often started in January (32%; 9/28) and February (50%; 14/28) and lasted 5–68 d. Impacts of mortality events on pigeon populations were indicated by Breeding Bird Survey and Christmas Bird Count abundance indices, which showed a decline in outbreak years compared to nonoutbreak years. Environmental conditions most associated with outbreak years included higher average temperatures between January and March, the period most associated with mortality events, and lower average precipitation in December just prior to mortality events. In Monterey County, events tended to occur in winters following higher acorn production of coast live oaks (Quercus agrifolia) in the fall. Weather and food abundance could be related to increased transmission or enhanced viability of Trichomonas spp. Although estimated mortality due to avian trichomonosis was highly variable across years, cumulative losses were substantial and likely to have a negative impact on population size.