Information on a total of 119 patients with echocardiographically detected right heart thrombi was collected by questionnaire. Two major thrombus types with different morphology, etiology and clinical significance were identified: (1) 48 patients had long, thin, extremely mobile thrombi which resembled a worm or a snake (type A); (2) 57 patients had more or less immobile, non-specific clots resembling left heart thrombi (type B). Type A patients had a high incidence of deep venous thrombosis and a low incidence of potentially thrombogenetic cardiac abnormalities. The reverse was true for group B. These observations and the peculiar worm-shape of type A thrombi suggest that most type A thrombi originate in peripheral veins, while most type B thrombi develop within the right heart chambers. Clinically, type A patients were a high-risk group; pulmonary embolism was the rule and was usually severe. Early (less than or equal to 8 days) thrombus-related mortality was excessively high (42%), including 13 deaths from pulmonary embolism, one from paradoxical peripheral embolism and six perioperative deaths. Type B thrombi were much more benign; pulmonary embolism was not uncommon (40%) but never fatal. Early thrombus-related mortality was only 4% (two peri-operative deaths). Fourteen cases could not be classified as A or B because their thrombi were highly mobile (= not B) but not worm-shaped (= not A). This small group was intermediate between groups A and B in all respects. An analysis of the relationship between therapy and outcome revealed that type B thrombi had a good prognosis irrespective of the treatment. In type A cases early thrombus-related mortality was much lower with surgery (27%) than with conservative treatment (54%). However, the selection of surgical cases was certainly very biased and it is not clear to what extent the better results of surgery are caused by patient selection. Thus the optimal management of these cases remains to be determined.