Invasive pulmonary aspergillosis was detected at postmortem examination of an 8-year-old Quarter Horse mare that had a history of diarrhea, laminitis, and bilateral nasal discharge. A diagnosis of Ehrlichia risticii infection was made on the basis of clinical signs of diarrhea and pyrexia, high serum antibody titer to E risticii (1:2,560), and multiple Salmonella-negative cultures obtained from fecal and colonic specimens. Possible pathogenic mechanisms contributing to the development of pulmonary aspergillosis secondary to enteric disease include mycotic invasion of the intestinal tract or immunocompromise and tissue devitalization secondary to severe enterocolitis. Exposure of the mare to large numbers of Aspergillus organisms may have been important in this case.