70 consecutive patients undergoing heart surgery for mitral valve disease were studied: at operation 6 of them exhibited left atrial thrombi (LAT). Dimension of the thrombi varied from a hazel-nut to an orange, 4 of them adhered to the posterior atrial wall, 2 of these obliterated the left atrial appendage, one partially and the other totally, invading also the left atrial cavity as far as mitral orifice. In 5 cases LAT appeared of old onset, possibly with recent apposition; in one case the thrombus was mainly recent. All the cases had been assessed preoperatively using M-mode and two-dimensional echocardiography: the diagnosis of LAT was made in 5 pts, the only thrombus missed was the one located in the left atrial appendage. Angiocardiography was performed in 4 pts, showing left atrial thrombi in one case. Two-dimensional echocardiography (2D E) demonstrated a high sensitivity by revealing LAT in 5 cases out of 6, with good definition of shape and location. The LAT appeared as echoproducing masse with well defined borders and "muscle" density in 4 cases; in 3 they were seen protruding into the atrial cavity and in one case they were seen located above the posterior mitral leaflet. M-mode revealed multiple echoes parallel to the posterior atrial wall in the first 3 cases, whereas in the fourth it provided no particular finding for the diagnosis. In the only case of recent onset LAT, 2DE showed a single strong echo, parallel to the posterior atrial wall both in long and short-axis views, separated from the atrial wall by an echo-free space of 1.5 cm. Similar features resulted at the M-mode echocardiography. In conclusion, 2DE with gray-scale has an high sensitivity for detecting LAT, in particular when thrombi are old and located in the left atrial cavity. Recent onset thrombi are more difficult to demonstrate and it is possible that the "fresh" component of an old thrombus is missed.