Background: Thyroid cytology reporting has become standardized with the introduction of the Bethesda System of Reporting Thyroid Cytology (BSRTC), linking up diagnostic categories with clinical management guidelines. The current thyroid cytology reporting format in Sri Lanka (TCRSL) is based on the original 'Thy' classification of the British Thyroid Association. The number of diagnostic categories have been increased in the BSRTC than in TCRSL (6 vs. 5 categories), with a possible adverse impact on inter-observer agreement (IOA). IOA of both reporting formats were thus compared to evaluate the appropriateness of looking beyond TCRSL in Sri Lanka, a country facing a significant thyroid cytology burden.Objective: To compare inter-observer agreement between the thyroid cytology reporting format in Sri Lanka and the Bethesda format. Methods: 100 thyroid aspiratesRetrieved smears were reviewed and classified using TCRSL and BSRTC by two pathologists independently. Results were tabulated using SPSS. Linear weighted κ and composite proportion of agreement values were used to evaluate the IOA of responses in both formats. Results:Linear weighted κ was 0.6138 (95% C.I. 0.4797 -0.7479) and the composite proportion of agreement was 68% for TCRSL. In BSRTC, linear weighted κ was 0.6795 (95% C.I. 0.563 -0.796) and the composite proportion of agreement was 70%.Conclusion: IOA for both reporting formats is in the substantial agreement range, with no significant difference in κ values. BSRTC shows a slightly higher composite proportion of agreement than TCRSL. Thus increased diagnostic categories of BSRTC have no significant adverse impact on the IOA in our setting.(