The effects of the addition of UHMWPE particles, of nominal (80 mu m) size, on the fracture toughness, flexural modulus and strength of composites made with dimethacrylate resins (60/40 wt/wt BisGMA-TEGMA) were investigated as a function of volume fraction of UHMWPE (0-60 vol%) and particle surface treatment. Interfacial shear strengths (tau) were measured via microbond shear strength tests using Spectra900 (TM) (UHMWPE) fibers and BisGMA-TEGMA beads. T increased by a factor of 4 compared with untreated UHMWPE, and surface treated particles improved the mechanical properties of the composite. Fracture toughness (K-IC) and flexural modulus (E) increased with increased volume fraction of UHMWPE, with maximum K-IC/E increases (at 60 vol%) of 238%/25% compared with the neat resin. SEM images showed debonding as well as yielding and fibrillation of the UHMWPE particles, suggesting that these were significant toughening mechanisms. (c) 2006 Published by Elsevier Ltd.