Abstract Objective Benign adrenocortical tumours are diagnosed in ∼5% of adults and are associated with cortisol excess in 30%-50% of cases. Adrenal Cushing's syndrome (CS) is rare and leads to multiple haematological alterations. However, little is known about the effects of the much more frequent mild autonomous cortisol secretion (MACS) on immune function. The aim of this study was to evaluate the haematological alterations in benign adrenocortical tumours with different degrees of cortisol excess. Design and methods We investigated 375 patients: 215 with non-functioning adrenal tumours (NFAT), 138 with MACS, and 22 with CS. We evaluated the relationship between the degree of cortisol excess and full blood count as well as multiple inflammation-based scores, including the neutrophil-to-lymphocyte ratio (NLR), the lymphocyte-to-monocyte ratio (LMR), and the systemic immune-inflammation index (SII). Results We observed a gradual and significant increase of leucocytes, neutrophils, and monocytes across the spectrum of cortisol excess, from NFAT over MACS to CS. Neutrophil-to-lymphocyte ratio and SII were significantly higher in both MACS and CS when compared to NFAT (P < .001 and P = .002 for NLR and P = .006 and P = .021 for SII, respectively). Conversely, LMR was lower in MACS and CS than in NFAT (P = .01 and <.001, respectively) but also significantly lower in CS compared to MACS (P = .007). Conclusions Neutrophil-to-lymphocyte ratio, SII, and LMR correlated with the degree of cortisol excess in benign adrenocortical tumours and were altered in patients with CS and MACS. These findings suggest that, similar to clinically overt CS, MACS also affects the immune function, potentially contributing to the MACS-associated comorbidities.