Background: Pump thrombosis (PT) is a dreaded complication after left ventricular assist device (LVAD) implantation. Problems with inflow cannula (IC) position may precipitate thrombus development. We sought to determine if IC position contributes to the development of PT.; Methods: We conducted a retrospective review of 76 HeartMate II LVAD implants. The angle of the IC (AIC) to the horizontal plane was measured on chest x-rays. Patients who developed PT (PT group) were compared to the remaining patients (control group).; Results: The mean age at implantation was 56 +/- 14 years, and 82% of the patients were male. Ten patients (13%) developed PT. Six (60%) required device exchange, and 4 (40%) were managed with anticoagulation and/or thrombolysis. The median AIC for all patients at implantation was 59 degrees (range, 38 degrees-98 degrees; 25th-75th interquartile range, 50 degrees-75 degrees). In the PT group, the median AIC was larger at the time of PT diagnosis compared to implantation (70 degrees vs 60 degrees, P = 0.005). In the control group, the median AIC was also larger at follow-up compared to implantation (61 degrees vs 58 degrees, P < 0.001) although to a lesser degree than in the PT group. No difference was seen in the median AIC between the PT group and the control group at implantation (60 degrees vs 58 degrees, respectively; P = 0.668) or at follow-up (70 degrees vs 61 degrees, respectively; P = 0.309). However, the median AIC at follow-up in the PT group was significantly larger than the median AIC at implantation in the control group (70 degrees vs 58 degrees, respectively; P = 0.014).; Conclusion: The HeartMate II LVAD IC position contributes to the development of PT. Regular monitoring of cannula position may help identify patients at risk for this problem.