This paper describes the clinical signs and histopathologic findings associated with an emergent disease associated with Trichomonas gallinae infections in free-ranging house finches (Carpodacus mexicanus) in California. Wet mounts were necessary to detect T. gallinae infections in house finches because classical clinical presentation, such as caseous stomatitis or ingluvitis, occurred in <25% of cases. Early detection was instrumental in preventing trichomonosis outbreaks in a high-density nursery (P < 0.0001). Detection before onset of clinical signs was critical. Despite treatment, ∼95% of house finches died within 24 hr of displaying signs of illness. In contrast, 58% of T. gallinae–positive house finches housed in a nursery survived if they received treatment before onset of clinical signs. Recurrent protozoal shedding in survivors was not evident.