Blood cells are involved in systemic inflammation in chronic obstructive pulmonary disease (COPD). We aimed to assess differences in leukocyte subsets and their ratios between COPD patients and healthy individuals as well as their association with disease severity, smoking status and therapy in COPD. One hundred and nine patients in the stable phase of COPD and 95 controls participated in the study. After blood sampling, white blood cells (WBC), neutrophils (NEUTRO), monocytes (MO), lymphocytes (LY) and basophils (BA) were determined on a Sysmex XN-1000 analyser, and ratios were calculated afterwards. White blood cells, NEUTRO, MO and BA were higher in COPD patients than in controls. Also, COPD patients had increased neutrophil to lymphocyte ratio (NLR), derived NLR (dNLR), monocyte to lymphocyte ratio (MLR), basophil to lymphocyte ratio (BLR), basophil to monocyte ratio (BMR) and monocyte/granulocyte to lymphocyte ratio (M/GLR). Smoking has an impact on leukocyte counts, with BA, BLR and BMR being higher in COPD smokers vs. ex-smokers. Patients with very severe COPD were distinguished from moderate COPD by NLR, dNLR and M/GLR. In addition, those parameters were associated with lung function and dyspnoea, and NLR and dNLR also with multicomponent COPD indices BODCAT and DOSE. Great potential of dNLR, NLR and M/GLR in identifying COPD patients was observed regarding their odds ratios (OR) of 5.07, 2.86, 2.60, respectively ( < 0.001). Common COPD therapy did not affect any of the parameters investigated. Leukocyte subsets and their ratios could be implemented in COPD assessment, especially in evaluating disease severity and prediction.