Chronic obstructive pulmonary disease (COPD) is the only major disease with increasing death rate. In COPD, progressive reduction in quality of life is closely related to the increasing limitation of airflow due to chronic bronchitis, cell hyperplasia, fibrosis, and irreversible lung damage. Signaling pathways involved in inflammatory processes in COPD and inflammatory response to therapy are unknown. Our aim was to isolate cells from induced sputum of COPD patients treated with formoterol or formoterol + tiotropium and assess enzymatic activity of histone deacetylases (HDACs) acetylated histone 4 (AcH4) and expression of inducible nitric oxide synthase (iNOS). HDACs are important in signal transduction and inflammation. iNOS is generating nitric oxide (NO) relevant to blood pressure regulation, inflammation and infections. Thirty stable COPD patients (21 males and 9 females, mean age 67 years) receiving 12 μg b.i.d. formoterol were assayed before and after 3 months add-on therapy consisting of 18 μg q.i.d. tiotropium. In all patients, spirometry, lung volumes, and DLCO were performed before and after tiotropium therapy and all patients were subjected to sputum induction. Sputum cells were isolated and processed to obtain cytosolic and nuclear fractions. HDAC activity was measured in nuclear fraction using colorimetric assay. Expression AcH4 and iNOS was quantified using Western blot. In patients receiving both drugs, FEV1 and lung volumes significantly improved compared with formoterol-only treated patients. Mean HDAC activity was slightly decreased (P < 0.05), while AcH4 levels and iNOS expression were significantly elevated in tiotropium-treated patients (increase by about 65 %; P < 0.01 and 77 %; P < 0.01 respectively). Our data show that beneficial effects of tiotropium in add-on therapy to formoterol may be related to altered histone signaling and increased iNOS expression.