A total of 25 CarboMedics bileaflet prostheses was studied in the immediate postoperative period and at 1 week, 1 month and 3 months after surgery. No evidence of rotation was found on visual comparison of valve orientation and there were no significant differences between successive studies for the distance between the septum and either the anterior (P = 0.92) or posterior leaflet (P = 1.00) of valves in the mitral position. The possibility of spontaneous rotation is a potential drawback not only of the CarboMedics prosthesis, but also of designs currently being developed. This study suggests that major rotation does not occur commonly.