In this study, a methacrylate-based orthopaedic bone cement was modified for use as a root canal sealer by changing the monomer from n-butyl methacrylate to tetrahydrofurfuryl methacrylate. The sealing and bonding capabilities, solubility and handling properties of the resin were evaluated. Seventy-three extracted mature human teeth with single canals were used. The teeth were randomly divided into two experimental groups each of 30 teeth; three control teeth were set aside for the leakage tests. Ten teeth were reserved for the scanning electron microscope studies. All the experimental teeth were root filled with gutta-percha and the resin sealer. The particle size of the polymer was limited in Group 1 to 75 microns or less and in Group 2 to 45 microns or less. The controls were filled with gutta-percha only. Silver nitrate solution was used as the leakage indicator. In Group 1 the mean linear penetration of the indicator was 1.10 mm +/- 1.04; four roots showed no leakage. In Group 2 the mean linear penetration of the indicator was 0.57 mm +/- 0.65; 12 roots showed no leakage. The test bodies showed no weight change after 1 and 3 months' storage in saline at 37 degrees C. The scanning electron microscope examination revealed a bond both between the resin and the dentine and between the resin and the gutta-percha. The working time of the new resin composition was normally about 50 min, and handling was found to be easy.