Objectives Vitamin D deficiency, as assessed by serum 25-hydroxyvitamin D (25[OH]D) concentrations, has been linked to markers of systemic inflammation in human and canine medicine. However, the relationship between vitamin D status and inflammation has not been previously investigated in cats. The aim of this study was to examine the relationship between serum 25(OH)D concentrations and leukocyte counts in hospitalised sick cats. Methods Serum 25(OH)D concentrations and haematology profiles were measured in 170 consecutive hospitalised sick cats. A binary logistical regression model examined the relationship between serum 25(OH)D concentration, age, sex, breed and neutrophil, monocyte, eosinophil and lymphocyte counts. Results Cats with neutrophilia had lower serum 25(OH)D concentrations than cats with neutrophil concentrations below the upper limit of the reference interval (RI). There were no differences in serum 25(OH)D concentrations in cats with monocyte, lymphocyte or eosinophil counts above their respective RI compared with cats with counts below the upper limit of the RI. Conclusions and relevance Hospitalised cats with a neutrophil count above the RI had lower vitamin D status. There is a need to establish whether lower vitamin D status is a cause or consequence of increased neutrophil counts.