The effect of ketotifen on myocardial contractility, functional status of patients with chronic heart failure (HF) due to ischemic heart disease (IHD) and type 2 diabetes in relation to chymase activity was investigated. It has been confirmed that HF due to IHD is associated with by significant increase of chymase activity, particularly in patients with concomitant diabetes. Correlation between the chymase activity, left ventricular ejection fraction and the results of 6-minutes walking test was detected. Ketotifen adding to standard therapy in HF patients induces inhibition of chymase activity accompanied with myocardial contractility improvement and 6-minutes walk test distance extension, including diabetic patients. The most significant ketotifen effect was noted in patients with initially higher chymase activity that defines differentiated approach to ketotifen prescription.