The fracture toughness K of 11 clinically used acrylic bone cements was studied in air at room temperature with single edge V-notched beam specimens. By driving the crack step-wise through the specimens, crack resistance curves ("R-curves") were recorded. One group of bone cements showed an increase of the fracture toughness with increasing crack length (including CMW1+G and several Palacos bone cements) whereas another group (including Simplex, SmartSet, Copal and some Palacos bone cements) did not exhibit an R-curve behavior. The plateau values for K ranged from 0.93 MPa√m (Simplex P) to 1.98 MPa√m (Palacos R+G). The observation of the crack growth with an optical microscope revealed some mechanisms influencing the crack growth like the formation of microcracks in the extended damage zone of the crack tip, the attraction of the crack by inclusions or the shielding of the crack tip by bridges in the wake of the crack. Furthermore, bone cements could be distinguished by the pattern of the path the crack followed during propagation. The crack pattern of CMW1+G provides a possible explanation of the distinct R-curve behavior of this cement.