The Utah-100 total artificial heart was designed to have increased reliability over the Jarvik-7 total artificial heart, achieve better fit, and minimize device associated thrombus formation, without decreasing the function. The Utah-100 heart was tested in 28 calves and 3 sheep. The smallest animal at the time of implantation weighted 54 kg. Mean survival duration was 78 days (range, 1-331 days), with 14 animals surviving longer than 60 days. Multiple organ function was maintained satisfactorily with the Utah-100 artificial heart, and mean plasma free hemoglobin values in the calves that survived longer than 100 days were less than 10 mg/dl. Hemorrhage was the main cause of death in animals dying within 30 days after implantation (5/13, 38%); infection was another primary cause of death or termination (4/31, 13%). Deaths due to mechanical failure occurred from valve or diaphragm failure in two cases, yielding a 91% reliability at a 90% confidence level for 60 days' support. No animal died because of driver or other technical failure. Utah-100 hearts showed superior antithrombogenicity in the connector and valve-related areas when compared with the results of the Jarvik-7 heart, which was also fabricated and implanted in our laboratory (p less than 0.01). With these test results, the authors anticipate that the Utah-100 heart will be a safe and effective device for interim use as a bridge to heart transplantation.