Aim This study was undertaken to examine whether a correlation exists between apical dye penetration and the clinical performance of root fillings.Methodology Apical dye penetration into 116 roots of human teeth that had been root-filled at least 6 months prior to extraction was tested in vitro using a vacuum technique and by measuring the length of dye penetration. Endodontic treatment was classified as clinically successful or unsuccessful and results for these groups were compared using analysis of variance and the Student's t-test. Positive and negative controls were used to test the experimental system.Results All controls performed as expected, Dye penetrated significant ly further in unsuccessful cases although the raw data suggested little difference, Overall, dye penetrated 99.5% of the specimens, indicating that the presence of dye in the canal is a poor indicator of whether the technique or material will succeed. However, the extent of dye penetration may be related to the clinical outcome.Conclusions Clinically placed root canal fillings do not provide an apical seal that prevents fluid penetration. The outcome of treatment cannot be predicted from the results of apical dye leakage studies.