Cushing's syndrome (CS) is a collection of signs and symptoms that develop as a result of high glucocorticoid levels in the body. The endogenous CS can be divided into two groups according to the cause of it's occurrence: ACTH dependent and ACTH independent CS. Pituitary corticotroph adenoma is the most common cause of ACTH dependent CS. In this case the disorder is called Cushing's disease. ACTH independent CS is most commonly caused by adrenocortical adenoma or adrenocortical carcinoma. The aim of this retrospective study was to compare haematological parameters between patients with CS and control groups. The study included 55 patients (46 woman and 9 men) with endogenous CS treated between May 2011 and January 2018 at the Division of Endocrinology, Department of Internal Medicine, University Hospital Centre Zagreb. Control groups consisted of subjects who were treated at the same Department for pituitary or adrenal gland tumors. Criteria for the choice of patients for the control groups were: absence of hypothalamic-pituitary-adrenal axis dysfunction and normal androgen levels. Female patients had higher values of E and Hb compared to women in the control group. This result was statistically significant (p<0,05). Men with CS had lower values of E and Hb compared to men in the control group, although this difference in male patients was not statistically significant. Different changes in E and Hb values between genders probably occur due to the inhibitory effect of glucocorticoids on gonadotropins and consequent hypogonadism in male patients. L and Ne values were higher in both women and men with CS compared to control groups. The result regarding L and Ne values was also statistically significant. This study showed that glucocorticoid excess is associated with high blood leukocyte count. Conversely, there is a gender difference in glucocorticoid effect on blood erythrocyte count.