Rupture of the posterior left ventricular wall following mitral valve replacement is a rare but fatal complication. Over a 10 year period from August 25 1980 to November 27 1990, we have experienced 6 such patients among 884 cases of mitral valve replacement with 4 deaths and 2 survivors. One patient had a type I rupture and another a type II rapture with the remaining four patients having suffered type III ruptures. All of the ruptures were dis covered intraoperatively enabling prompt reinstitution of the cardiopulmonary bypass and subsequent cardioplegic arrest prior to repair. Overzealous removal of calcified valve leaflets seemed to be responsible for the single type I rupture, and untethering of the so called ventricular loop appeared to be the main mechanism responsible for the type III ruptures. The single type II rupture that had occurred seemed to have been caused by inadvertent laceration of the papillary muscle with resultant rupture of the posterior LV wall at the base of the papillary muscle. Among the type III ruptures, 2 patients required intraaortic balloon pump[IABP] support only for mechanical assistance and 1 patient required both the IABP and the Biomedicus LV assist device for successfull weaning following repair of the LV rupture Another patient with a type II rupture also required the circulatory assistance of both the IABP and the bio-medicus LV assist device for weaning from the bypass. Attention to meticulous technical considerations such as avoiding over aggressive removal of heavily calcified valvular tissue, preservation of as much mural leaflet tissue and chordal stuctures as possible seemed helpful in preventing this catastrophic complication from occurring. Fusion and fibrous stricture of the chordal structures appeared particularly conducive to the type II ruptures as a result of the increased susceptibility to papillary injury during operation.