The role of vitamin E deficiency in the development of retinal pigment epithelial dystrophy was investigated in 11 cocker spaniels and four other dogs. The concentration of α-tocopherol was measured by high performance liquid chromatography in plasma samples obtained from the affected dogs and from 28 ophthalmoscopically normal, healthy control dogs. The mean (sd) plasma α-tocopherol concentration in the normal dogs was 20.2 (7.1) μg/mI, compared with 1.14 (0.67) μg/mI in the 11 affected cocker spaniels. The difference between the two groups remained highly significant when the α-tocopherol concentrations were expressed relative to the concentrations of the plasma lipids cholesterol and triglycerides. Low plasma concentrations of α-tocopherol were observed in the four affeded dogs of other breeds, but the finding was not so consistent. The plasma lipid concentrations were normal in the affecded dogs. The deficiency of α-tocopherol in the affeded dogs appeared to be primary, because there was no clinical, biochemical or pathological evidence of underlying disease, or any indication of a dietary deficiency which might have contributed to the low concentrations of α-tocopherol.