To evaluate the effects of left ventricular hypertrophy in the development of myocardial ischaemia as detected by thallium-201 exercise scintigraphy, we studied 150 patients by two-dimensional echocardiography, SPECT thallium-201 scintigraphy, and coronary arteriography. Patients were divided into four groups according to the presence or absence of left ventricular hypertrophy and coronary artery disease. There were 62 patients with coronary artery disease and 28 with left ventricular hypertrophy. Left ventricular mass index ranged from a mean of 91.2 g m-2 in the no LVH no CAD group, to a mean of 150.6 g m-2 in the LVH CAD group. For the whole group, the sensitivity of thallium for the detection of coronary artery disease was 82%, specificity 75%, positive predictive value 70%, and negative predictive value 86%. The proportion of patients with abnormal thallium was highest (100%) in the group with left ventricular hypertrophy and coronary artery disease. The second largest group with abnormal perfusion by thallium imaging comprised patients with coronary artery disease and no left ventricular hypertrophy (78%). Both coronary artery disease and left ventricular mass index were demonstrated to be independent predictors of myocardial ischaemia in a multivariate regression model (P = 0.01). We conclude that left ventricular hypertrophy sensitizes the myocardium for the development of ischaemia.