The critical difference, which may help to judge whether the difference between two consecutive analytical results may be safely ascribed to natural variation or not, was calculated for 12 clinical chemical components determined in blood samples collected once a week for 5 consecutive weeks from 19 clinically healthy Red Danish dairy cows. For each clinical chemical component, the total variance of the analytical results was divided into the component of variance between cows (S2Inter), the component of variance for weeks within cows (S2Intra) and the component of variance for measurements (S2Anal) using nested analysis of variance. The critical difference calculated in absolute values from S2Intra and S2Anal was 0.15 mu kat per 1 for alanine aminotransferase, 0.55 mu kat per 1 for aspartate aminotransferase, 0.57 mu kat per 1 for alkaline phosphatase, 0.14 mu kat per 1 for gamma-glutamyltransferase, 1.95 mu kat per 1 for creatine kinase, 2.23 mmol per 1 for urea, 22 mu mol per 1 for creatinine, 2.4 g per 1 for albumin, 10.0 g per 1 for serum protein Total, 0.71 mmol per 1 for glucose, 0.54 mmol per 1 for calcium and 0.25 mmol per 1 for magnesium. These critical differences may be used as guidelines to evaluate the difference between two consecutive analytical results in cows. However, the analytical results should not be assessed by the critical differences alone, but should also be compared with the corresponding reference intervals.