Laying hens (n = 2267) ranging in age from 2 to 4 yr in a study evaluating ovarian cancer prevention were necropsied. Those that died or were culled during the 2-yr study (n = 1591) were necropsied weekly to determine the most probable cause of death or culling and cancer status. Hens surviving until the end of the study (n = 676) were euthanized and necropsied. Hens necropsied before and after a hen with proventricular intussusception served as cohorts (n = 38). Nineteen hens (13 dead, 6 culled) had intussusceptions of the proventriculus into the ventriculus. Mean age of affected hens was 154 wk (range 110–204 wk). None of the hens in the study had an intestinal intussusception, and none of the hens euthanized at the end of the study had a proventricular intussusception. Hens with proventricular intussusceptions were severely emaciated; mean body weights were 1040 and 1736 g for affected and cohort hens, respectively. Necropsy findings included prominent keel, marked muscle atrophy, generalized serous atrophy of fat, no visible proventriculus, esophagus directly entering the ventriculus, and an enlarged, spherical, firm ventriculus, which contained an invaginated, swollen, diffusely ulcerated proventriculus. Eighteen affected hens were anovulatory (94.7%) compared to 27 cohorts (71.1%). Severe, diffuse necrosis and ulceration of the proventricular mucosa was confirmed microscopically, but no etiologic agent was identified. In conclusion, proventricular intussusception of undetermined etiology was identified as a cause of sporadic emaciation, culling, and mortality in older laying hens.