We have compared the diagnostic accuracy of left coronary and laevo-phase pulmonary angiograms in detecting left atrial thrombi in 27 consecutive patients with rheumatic mitral stenosis who underwent both these procedures prior to open-heart surgery. In 10 patients, both procedures were 'positive' for thrombus, confirmed at subsequent surgery in all instances (true positive). Both procedures were negative for thrombus in 11 patients, and none of these patients showed thrombi at subsequent surgery (true negative). Left coronary angiography only was positive for thrombus in one patient in whom no thrombus was found at surgery. Laevo-phase pulmonary angiography only was positive for thrombus in five patients, two of whom had thrombus at subsequent surgery. The sensitivity, specificity and predictive accuracy of left coronary angiogram were 83.3% and 93.3% and 90.9% and that of laevo-phase pulmonary angiogram 100%, 80% and 80% respectively. Laevo-phase pulmonary angiograms showed higher sensitivity and left coronary angiograms showed higher specificity for angiographic diagnosis. However, the differences were found to be statistically insignificant. Angiography is a reliable method for detecting left atrial thrombi if both left coronary and pulmonary angiograms are performed and both procedures are positive or negative for thrombus.