Purpose: The aim of this study was to evaluate the cleaning efficacy of interdental brushes with different stiffnesses, e.g. soft and hard interdental brushes with identical brush diameter. Materials and Methods: Cylindrical soft and hard interdental brushes with diameters of 2,3 and 5 mm each were tested. Sixteen extracted human molars were fixed in split cast models to simulate eight interdental spaces. After coating the teeth with a dye to simulate plaque, digital photographs were taken from the proximal surfaces in a highly standardised set-up. The teeth were repositioned and the proximal surfaces were cleaned in a standardised manner. Post-brushing digital photographs were taken as before. After digital subtraction, the cleaned area was measured by pixel count and the relative cleaning efficacy was calculated. Results: The cleaning efficacy values of soft and hard interdental brushes of corresponding size in extra-small, small, medium and large interdental spaces as well as overall showed no statistically significant difference. In small, medium and large interdental spaces, increasing brush diameters resulted in higher cleaning efficacy; these differences were statistically significant. Irregular values were seen in extra-small interdental spaces. Conclusion: Both hard and soft interdental brushes cleaned the proximal tooth surfaces effectively. The filament stiffness had no statistically significant influence on the cleaning efficacy. Keywords: cleaning efficacy, filament stiffness, interdental brushes, mechanical plaque control