Purpose: To evaluate the effect of slice thickness on diagnostic accuracy in Digital Breast Tomosynthesis (DBT). Method: Two readers retrospectively interpreted 150 DBT (125 normal and 25 pathology-proven cancer) cases scanned between October 2017–November 2020. The DBT studies were randomised and reviewed independently by the two readers. DBT studies were reviewed using a standard protocol (1 mm slices, no overlap and synthetic 2D-mammography (SM)) and an experimental protocol (10 mm slabs, 5 mm overlap and SM). Any abnormality and BIRADS scores were recorded by each reader. Sensitivity, specificity, interobserver and intraobserver agreement were calculated (Cohen’s Kappa κ). For diagnostic accuracy, the reference standard was histopathology or a normal mammogram at 2 years. Results: The sensitivity and specificity for reader 1 and 2 for cancer detection was reader 1 (97% and 79% for the standard protocol, 97% and 76% for the experimental protocol) and reader 2 (97% and 74% for both protocols). Reader 1 had 97.6% intraobserver agreement (κ .95) and reader 2 had 96.4% intraobserver agreement (κ .92) when assessing the standard and experimental protocols. There was 90.5% agreement between the readers for the standard protocol (κ .80). There was 90.9% agreement between the readers for the experimental protocol (κ .81). Of the 25 DBT studies with pathology-proven cancer, one cancer was missed by both readers using both protocols. Conclusion: The diagnostic accuracy was similar between the standard and experimental DBT protocols, demonstrating excellent interobserver and intraobserver agreement. This suggests 10-mm thick slabs can potentially replace 1-mm thin slices in the interpretation of DBT.