© 2017, Turkish Biochemistry Society. All rights reserved.Background: The neutrophil-lymphocyte ratio (NLR) is a good marker of malignant diseases. The aim of this study was to investigate NLR as a diagnostic marker of thyroid cancer in patients with malignant thyroid diseases compared to those with benign thyroid diseases. Methods: Two hundred and fifty three patients were divided into two groups: benign and malignant thyroid disease patients. Malignant thyroid disease group was not sub-grouped further into differentiated and undifferentiated. The preoperative complete blood count and biochemical parameters, including NLR, were evaluated. Results: The mean neutrophil count was found to be significantly higher, whereas the mean lymphocyte count was found to be significantly lower in the malignant group which includes 40 of the patients. As a result, NLR was significantly higher in the malignant group than in the benign group (p < 0.001). Conclusion: This is the first study to show the value of NLR as a marker for malignant thyroid diseases including all subtypes.