The relationship between body dry weight (W) and shell length (L) of blue mussels, Mytilus edulis, can be expressed by the condition index (CI = W/L3) which varies from population to population and during the year. Here, we examine the influence of CI on the relationships between maximum filtration rate (F, l h−1), W (g), and L (mm) as described by the equations: FW = aWb and FL = cLd, respectively. This is done by using available and new experimental laboratory data on M. edulis obtained by members of the same research team using different methods and controlled diets of cultivated algal cells. For all data, it was found that FW = 6.773W0.678 and FL = 0.00135L2.088 which are very similar to equations for mussels with ‘medium condition’ (CI = 4–6 mg cm−3): FW = 6.567W0.681 and FL = 0.00150L2.051, with b- and d-values within a few percent of the theoretically expected of 2/3 and 2, respectively. Further, based on the present data, we propose a correction factor expressed by the empirical relation FW/FL = 0.3562CI2/3 which implies that FW tends to underestimate the actual filtration rate (FL) when CI < 4.70 and to overestimate the filtration rate when CI > 4.70.