The results of Japanese studies on the clinical efficacy and safety of pimobendan in the treatment of heart failure are summarized in this paper. In patients with acute heart failure, a single oral 2.5 mg dose of pimobendan produced a significant increase in cardiac index (27.9%) and stroke volume index (24.4%), as well as a significant decrease in pulmonary capillary wedge pressure (-21.3%). A single oral 5.0 mg dose of pimobendan increased anaerobic threshold during exercise (7.3%). In a multicentre, placebo-controlled, double-blind trial, 8 weeks of treatment with pimobendan 2.5 mg b.i.d., which was suggested to be the optimal dose in a preceding dose-finding study, definitely improved symptoms and signs of heart failure in 52.6% of patients, and New York Heart Association function class in 64.1% of patients. This regimen was also associated with a significantly greater increase in physical activity than placebo. Furthermore, another double-blind, placebo-controlled study suggested that pimobendan 1.25 mg and 2.5 mg b.i.d. may have long-term beneficial effects on morbidity and quality of life in patients with chronic heart failure. Adverse reactions developed in 26 of 241 (10.8%) patients receiving multiple doses of pimobendan, but none of the reactions were serious. It is concluded that pimobendan may be useful in the treatment of patients with heart failure.