Bioprosthetic heart valves have several modes of failure. Tissue degeneration and calcification are the major modes of failure with the highest focus of attention, however pannus formation can also be problematic. We studied the effect of a new tissue technology with the absence of any glutaraldehyde-based storage solution and a stable aldehyde capping process on pannus formation. Using a juvenile sheep model of mitral valve replacement, valves with the new tissue technology were compared to control valves with contemporary bovine pericardial tissue, regarding pannus formation. Valves were implanted for either a 5- or 8-month period. Explanted valves were examined macroscopically and histologically. Histological observations were made by an independent pathologist, blinded to group identity. Pannus area measured macroscopically on the test valves was significantly lower than the pannus on the control tissue. This was confirmed on the histological samples, where the total pannus overgrowth was significantly lower in the test group compared to the control. The new tissue technology leads to less pannus formation. This may beneficially influence both short- and long-term valve behavior of bioprosthetic valves.