A six week old Fell Pony filly presented to the Cornell University Hospital for Animals Large Animal Medicine service with a chief complaint of chronic progressive anemia. The filly's packed cell volume had been monitored by the referring veterinarian since birth as part of a study of Fell Pony Syndrome. The packed cell volume was normal at birth, but began to steadily decline two weeks prior to presentation. Foals affected by Fell Pony Syndrome are normal at birth, but at approximately four weeks of age begin to develop severe non-regenerative anemia due to ineffective erythropoiesis and opportunistic infections secondary to B lymphopenia. Recent work has suggested circulating IgM levels in Fell Pony foals after 4 weeks of age are positively correlated with peripheral B-lymphocyte concentrations and that measurement of circulating IgM levels after 4 week of age may be a more accessible predictor of Fell Pony Syndrome than B-lymphocyte measurement.