Enoximone (MDL 17,043), a newly synthesized imidazole derivate, has been shown to possess both positive inotropic and vasodilating properties. Sixteen patients with congestive heart failure were allocated to receive either enoximone, 50, 100 or 150 mg, or placebo, each given 3 times daily for 4 weeks, to investigate the dose-related efficacy and tolerability of oral enoximone. Symptom-limited exercise capacity improved in 5 of 10 patients in the enoximone group. The ejection fraction increased from 28% to 36% after 4 weeks, to 36% after 8 weeks and to 35% after 12 weeks in the enoximone group. Exercise duration and ejection fraction did not change in the patients in the placebo group. With enoximone, heart rate, blood pressure, Holter monitoring and laboratory tests showed no significant drug-related changes. The addition of oral enoximone to existing therapy with digitalis and diuretics may improve the clinical condition and left ventricular function in patients with congestive heart failure. Enoximone shows clinical efficacy at dosages of 50 mg and 100 mg 3 times daily. With the higher dosage, unwanted side effects increased but efficacy did not. Enoximone did not increase ventricular ectopy in the doses given.