Sixty-two patients were investigated for chronic intravascular hemolysis from 3 months to 6½ years following aortic valve replacement; 27 patients had Starr-Edwards prostheses and 35 fascia lata grafts.The findings are suggestive of a significantly higher incidence and degree of hemolysis in patients with Starr-Edwards prostheses compared to those with fascia lata grafts. In both groups regurgitation was an important factor in producing hemolysis. In the absence of regurgitation fascia lata grafts do not produce significant hemolysis, while Starr-Edwards valves traumatize the red cells even in the absence of regurgitation. Starr-Edwards valves of small size are associated with hemolysis more often than larger ones are. Turbulent flow due to the obstructing ball is the probable mechanism of hemolysis in these patients. In most of the patients the degree of hemolysis appears to remain at the same level of activity. In rare instances, however, the hemolysis may regress. The mechanism for this is not clear. In a small number of patients, also, progressive hemolysis may necessitate removal and replacement of the valve.