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The patients with so-called syndrome X experience true myocardial ischaemia in spite of their anatomically normal coronary arteries. The pathogenetic mechanism of the effort-induced ischaemia in these patients should be obviously different from that of patients with atherosclerotic coronary artery lesions. An inadequate coronary dilatory capacity in response to increased oxygen demand has been proposed as mechanism responsible of ischaemic response to effort (Canon et al., 1983; Canon et al., 1985). Since diastolic duration expressed as percentage of the cardiac cycle has implications with the myocardial oxygen demand/supply ratio (Boudoulas et al., 1982; Maseri et al., 1977), the purpose of this study has been to evaluate the behaviour of this parameter at the outcome of the ischaemic electrocardiographic changes during stress testing in the patients with syndrome X.