The mountain chicken frog (Leptodactylus fallax) is the largest endemic amphibian species in the Western Hemisphere. Since 1998, this critically endangered species has been maintained as a European Endangered Species Programme, but low breeding success and a high mortality rate threaten the sustainability of the captive frog population. In the current study, we analyzed gross and histopathologic postmortem information from 212 mountain chicken frogs that died in European zoological collections from 1998 to 2018. Thin body condition was the most commonly reported finding across all submissions, observed in 125 frogs. The gastrointestinal and urinary systems were reported to have the highest prevalence of pathologic findings on gross and histopathologic examination. Inflammatory disease was the most frequent diagnosis after histopathologic examination of relevant tissues, with intestinal inflammatory disease (n = 76) followed by tubulointerstitial nephritis (n = 26) being the most commonly reported. Neoplasia was reported in 42 of 212 (19.8%) frogs, all of which were adults. A defined cause of death, or reason for euthanasia, was proposed for 164 of 212 (77.4%) frogs, with inflammatory diseases processes (74 of 212; 34.9%) most commonly implicated. Intestinal adenocarcinoma, seemingly restricted to the colon, caused the deaths of 31 adult frogs. Further investigations to determine factors contributing to the high incidence of inflammatory disease processes and neoplasia are advocated to improve the health and sustainability of the captive mountain chicken frog population.