Objective: To evaluate the percentage of filled area of oval-shaped root canals. Materials and methods: Thirty-four mandibular incisors, 42 mandibular premolars and 50 mandibular molars (distal roots only) (n = 126 teeth) were resected 1 and 5 mm from the apex and the cross sections evaluated. All treatments were performed by undergraduate students, who were not aware of the cross-sectional shape of the canal. Obturation was performed using the lateral compaction technique of gutta-percha with sealer. Results: The findings demonstrated that although all treated root canals were oval-shaped at 5 mm short, the vast majority of them were round at the 1-mm level. The percentage of the total filling area at the 1-mm level was 79% (range: 20-100%) for incisors, 89% (range: 46-100%) for premolars, and 97% (range: 60-100%) for the molar distal root. Correspondent values for the 5-mm level were 92% (range: 52-100%) for incisors, 99% (range: 86-100%) for premolars, and 96% (range: 78-100%) for molars. Gutta-percha was always the major component of the filling mass. At the 1-mm level, the mean proportion of gutta-percha ranged from 66% to 79%, while at the 5-mm level it ranged from 62.5% to 76%. Conclusion: The findings confirmed that the quality of the root canal filling is compromised in ovalshaped canals, and suggested that this may be more likely a limitation of the technique than a matter of the operator's expertise. Keywords: endodontic treatment, gutta-percha, lateral compaction technique, oval root canals, pre-clinical training, root canal filling