The aim of this study was to determine the frequency, significance and prognosis of small, abnormal, strand-like echos observed by early transoesophageal echocardiography after mitral mechanical valve replacement with hemi-disc prostheses. One hundred and twenty nine consecutive patients operated between October 1988 and June 1992 underwent transoesophageal echocardiography on average 15 +/- 7 days after surgery. A second transoesophageal echocardiography was performed in 52 patients on average 8 months after the first postoperative examination. The frequency of small strand-like echos and of non-obstructive thromboses of the valve at the initial transoesophageal examination was 43% and 8.5% respectively. A multivariate analysis showed that the only independent predictive factor for prosthetic valve strands was spontaneous intra-atrial contrast (p < 0.01). The presence of strands was significantly related to the prevalence of early thrombo-embolic events (confirmed non-obstructive valve thrombosis and systemic embolism). Strands were observed in 80% of cases with early thromboembolic complications compared with only 38% of cases with no early thromboembolic events (p < 0.04). Univariate analysis showed that the protamine/heparin ratio at the end of cardiopulmonary bypass and the percentage of ineffective postoperative heparinisation were higher in patients with these small, abnormal echos (p < 0.05 and p < 0.001, respectively). These appearances disappear at long-term transoesophageal echocardiographic control examinations in about half the cases. The authors conclude that the majority of these small, abnormal echos are thrombotic in nature and are associated with a higher thromboembolic risk, justifying effective anticoagulation in the first postoperative hours whenever possible, considering the risk of bleeding.(ABSTRACT TRUNCATED AT 250 WORDS)