This report describes the clinical progression of a Thoroughbred mare with common variable immunodeficiency (CVID). Equine CVID is a primary immunodeficiency characterized by late-onset B cell lymphopenia or depletion, hypo- or agammaglobulinemia, and recurrent fevers due to bacterial infections. The mare in this report had a history of chronic weight loss, intermittent respiratory disease, fevers, and episodes of colic. She presented to Cornell University Hospital for Animals for assessment of liver disease.