&NA; This study assessed the accuracy of using a two‐dimensional principal stress analysis compared to a three‐dimensional analysis in estimating peak turbulent stresses in complex three‐dimensional flows associated with cardiac prostheses. Three‐component, coincident laser Doppler anemometer measurements were obtained in steady flow downstream of three prosthetic valves: a St. Jude bileaflet, Bjork‐Shiley monostrut tilting disc, and Starr‐Edwards ball and cage. Two‐dimensional and three‐dimensional principal stress analyses were performed to identify local peak stresses. Valves with locally two‐dimensional flows exhibited a 10‐15% underestimation of the largest measured normal stresses compared to the three‐dimensional principal stresses. In nearly all flows, measured shear stresses underestimated peak principal shear stresses by 10‐100%. Differences between the two‐dimensional and three‐dimensional principal stress analysis were less than 10% in locally two‐dimensional flows. In three‐dimensional flows, the two‐dimensional principal stresses typically underestimated three‐dimensional values by nearly 20%. However, the agreement of the two‐dimensional principal stress with the three‐dimensional principal stresses was dependent upon the two velocity‐components used in the two‐dimensional analysis, and was observed to vary across the valve flow field because of flow structure variation. The use of a two‐dimensional principal stress analysis with two‐component velocity data obtained from measurements misaligned with the plane of maximum mean flow shear can underpredict maximum shear stresses by as much as 100%. ASAIO Journal 1996;42:154‐163.