Systematic transoesophageal echocardiography after the 10th day of mitral valve replacement with a mechanical prosthesis has enable diagnosis of several abnormal conditions: thrombi, strands and paravalvular leaks. The aim of this prospective study was to determine the prevalence of these conditions by biplane transoesophageal echocardiography in the first 24 postoperative hours. Transthoracic and transoesophageal echocardiography was performed on average 12 +/- 3 hours after coming out of the surgical block in 77 consecutive patients who underwent mitral valve replacement with a mechanical prosthesis. Nine patients (11.7%) had appearances of thrombi or strands in the left atrium or auricle. These small thrombi (1 to 1.5 cm2) were not obstructive. No embolic events were observed in the first month of these patients. Spontaneous contrast was seen in the left atrium of 31 patients (40%). The factors associated with the presence of thrombus or strands were advanced age (p = 0.02), presence of spontaneous contrast (p = 0.02) and more dilated left atrium (p = NS) Paraprosthetic leaks were seen in 11 cases (14.3%). In 10 cases, the regurgitant jets were narrow at their origin with little extension into the left atrium. Only one patient had severe regurgitation associated with raised transprosthetic pressure gradients. Cases with paravalvular leaks had a lower incidence of spontaneous contrast (3.2% vs 21.7%) and no thrombosis. Transoesophageal echocardiography demonstrated spontaneous contrast, thrombi and strands in the initial hours following implantation of a mechanical mitral valve prosthesis. The prevalence of these appearances, comparable to that of the results reported with later investigations, underlines the importance of effective anticoagulation from the fist postoperative hours.