The purpose of this study was to compare quantitatively the density of gutta-percha root canal fillings produced by cold lateral condensation with those produced by an ultrasonically energized spreader in a warm lateral condensation technique in artificial root canals. Thirty-degree simulated root canals in 30 transparent acrylic blocks were instrumented by using rotary files. The blocks were weighed. The canals were obturated with gutta-percha using cold lateral condensation without sealer and then weighed again. Two applications of warm lateral condensation were then performed on the same 30 canals using the UES. The blocks were weighed once more after each additional UES obturation treatment. The first heat application produced a 26.92% increase in weight over lateral condensation, and the second heat application produced an additional 5.95% increase. A one-way repeated measures ANOVA followed by the Tukey-Kramer test revealed that both applications of warm lateral condensation produced significant (p < 0.001) increases in weight. Results indicate that warm lateral condensation using a ultrasonically energized spreader results in denser gutta-percha fills by weight than cold lateral condensation.