During a two-year period (August, 1971, to July, 1973) 1 Starr-Edwards Model 2400 and 135 Starr-Edwards Model 2320 aortic prostheses were inserted without postoperative antiocagulant therapy. Seven of these valves (including the Model 2400 prosthesis) have been replaced because of substantial transvalvular gradients resulting from fibrous overgrowth of the valve orifice. Symptoms associated with aortic stenosis occurred an average of nine months after the initial operation. Six patients had severe anemia (packed cell volume less than 30%) and hemolysis (serum lactic dehydrogenase greater than 900 units). Gradients across the prostheses ranged from 66 to 105 mm Hg with a mean of 87 mm Hg. One valve was size 8A, 3 were 10A, and 1 was 12A. Fibrous ingrowth was not observed in an earlier group of 23 patients receiving the Model 2310 or Model 2320 prosthesis who were placed on long-term anticoagulant therapy. These findings, coupled with a 9% incidence of thromboembolism at one year, indicate that patients having these prostheses inserted should receive long-term anticoagulant therapy.