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Abstract Background In cows with acute toxic mastitis (ATM), the leukogram aids in the assessment of the severity of disease. The goal of our study was to compare the leukogram of 158 cows with ATM (cases) and 168 clinically healthy cows (controls). We hypothesised that the leukograms of surviving and non-surviving cows differ and that there are variables of the leukogram with sufficient prognostic potential to be used in the decision to treat or not to treat a cow with ATM. The cows were examined clinically and underwent haematological and biochemical examination of blood and bacteriological culture of milk samples. Results All cows with ATM had a poor appetite or anorexia, and 34 cows (21.5%) were recumbent. A single quarter was affected in 119 cows (75.3%), two quarters in 37 cows (23.4%) and three quarters in two cows (1.3%). Bacteriological culture showed Gram-negative pathogens in 100 cows (63.3%), Gram-positive in 15 (9.5%) and yeast in 4 (2.5%). The median total leukocyte count of cases was 4300 cells/µL (interquartile range = 2300–8200/µL), which was significantly lower than 8000 cells/µL (6525–9300/µL) in controls. Except for band neutrophils and metamyelocytes, the counts of all components of the leukogram were lower in cases compared with controls. Significantly more cows with ATM had leukopenia (60.1 vs. 4.1%) or leukocytosis (10.1 vs. 3.0%) than controls. Diseased cows had significantly lower segmented neutrophil counts than controls (860 vs. 2598 cells/µL), and 69.5 and 17.3%, respectively, had counts below the reference interval. Cases had increased band (77.3%) and metamyelocyte (25.0%) counts compared with controls (0.6 and 0%, respectively). In diseased cows, eosinopenia occurred in 66.4% (controls, 1.8%), monocytopenia in 40.6% (4.2%) and lymphopenia in 60.2% (1.8%). Twenty-one diseased cows (16.4%) had a regenerative and 57 (44.5%) had a degenerative left shift. The median neutrophil-to-lymphocyte ratio was 0.97 in diseased cows and 0.63 in controls. Toxic changes in neutrophils including cytoplasmic basophilia and vacuolisation were seen in 101 (91.8%) of 110 blood smears of diseased cows. The leukogram of the surviving and non-surviving cows did not differ significantly, and the hypothesis was rejected. Conclusions ATM results in severe changes in the leukogram particularly leukopenia, lymphopenia, and degenerative left shift. The hypothesis that the leukograms of surviving and non-surviving cows differ was rejected. The leukogram has not sufficient prognostic potential to be used in the decision to treat or not to treat a cow with ATM.