An increased incidence of cataract and fatty liver in plains viscachas (Lagostomus maximus) was noted for many years at the Zurich Zoo (Switzerland). Based on elevated serum fructosamine and glucose, diabetes mellitus was diagnosed; and these parameters normalized when the diet of the animals was changed from a low-fiber to a high-fiber diet. In this present study, 177 necropsy reports from before and after the diet changes were evaluated for the incidence of cataracts and fatty liver. Sixteen of 56 animals (29%) that were born before the diet change developed cataract. In contrast, only two of 121 animals (1.65%) that were born after the diet change developed cataract. The prevalence of cataract and fatty liver in animals born after the diet change (1% and 0%, respectively) was significantly lower than in animals born before (9% and 6%, respectively) the diet change. The results suggest that the plains viscachas at the Zurich Zoo were affected by an alimentary-induced diabetes mellitus (type 2) before the diet was changed.