Pierce-Donachy ventricular assist devices (VADs) were used to support 54 patients. None of the 27 patients supported less than 4 days had any visible thrombi in the VAD at explant. Of the 27 patients supported longer than 4 days, nine patients had visible thrombi. In this group of 27 patients, 19 received VAD support pending myocardial recovery, and anticoagulation consisted of dextran (4-15 days) and heparin during the weaning phase (1-2 days). The other eight patients had VADs implanted as bridges to cardiac transplantation. These patients also received dextran postoperatively (1-5 days), but were switched to a regimen of oral warfarin and dipyridamole. Duration of support for the nine patients in whom thrombi were found ranged from 4-27 days (mean, 12 days). The etiology of thrombus was probably related to mechanical problems in four patients, inadequate anticoagulation in two patients, sepsis in two patients, and unknown causes in one patient. Four of these patients suffered cerebral or peripheral injuries, which were probably embolic in two, and possibly embolic in two. One of these four patients survived, and two patients with thrombi but no emboli survived. One patient suffered a stroke, but no evidence of thrombi was found. Our incidence of thrombus with this VAD was 17%. Thrombi were related to inadequate anticoagulation, interrupted flow, or sepsis, but not to duration of support.