It has been clearly established that ischemic heart disease, hypertension and ageing affect diastolic function before any change is observed in contractile function. Though an increasingly recognised clinical entity, cardiac failure with normal systolic function still does not have any specific treatment. Phosphodiesterase inhibitors which increase AMPc, in addition to their inotropic and vasodilator effects, accelerate relaxation. Major and isolated abnormalities of relaxation have been demonstrated in vitro in non necrosed tissues of both the dilated and hypertrophic forms of advanced cardiomyopathy. The myocardium seems unable to restore rapidly the low cytosolic calcium concentrations required for the deactivation of the contractile proteins. The underlying mechanisms are probably very complex but a deficit in AMPc production has been demonstrated in very advanced stages of cardiomyopathy. In ischemia, however, the abnormalities of relaxation seem to be directly related to a defect in free energy production inhibiting the sarcoplasmic reticulum calcium pump. If abnormalities of relaxation due to ischemia and those due essentially to a passive mechanism are excluded, phosphodiesterase inhibitors would seem to have pharmacological effects likely to improve diastolic function. Clinical studies confirm the beneficial effects of Milrinone and Enoximone on relaxation and the rapid phase of diastolic filling, both in acute and chronic studies. However, it has not yet been clearly established whether improved diastolic function is due to a direct action on the myocardium or an indirect action due to improved conditions of load. In order to determine the specific effects of phosphodiesterase inhibitors on diastolic function, further research is required.(ABSTRACT TRUNCATED AT 250 WORDS)