A large fluctuation in normal serum vitamin E (alpha-tocopherol) concentrations (> 2 μg/ml) has beendemonstrated in normal horses. The purpose of this study was to verify the fluctuation in serum vitamin E inhorses with deficient levels (< 1.5 μg/ml) and to assess the diagnostic value of a single serum sample todetermine the vitamin E status in a deficient horse. Serum vitamin E levels were monitored over a period of24 hours in 6 normal horses and in 22 horses with clinical signs that may be related to vitamin E deficiency.The serum vitamin E levels varied widely within each horse, and the coefficient of variation (CV) wassignificantly larger in the deficient horses (mean CV: 41%), compared to the horses with normal levels (meanCV: 14%). In a small proportion of the deficient horses, the vitamin E levels varied from deficient to normalwithin 24 hours. The sensitivity of a single sample compared to the “true value” determined by the averageof the samples, was 91%. The sensitivity in case of 2 serum samples increased to 97%. These findings suggestthat a single serum value does not always provide reliable information about the true vitamin E status in adeficient horse and analyzing 2 serum samples allows a more accurate estimation of the vitamin E status. Theconclusion therefore is that when a serum vitamin E value > 1.5 μg/ml is found in clinically suspected horses,a second serum sample should be evaluated.