Background: The size of thyroid nodules as measured by ultrasound (ultrasound size, USS)is routinely used in clinical decision-making. Reports of discrepancy between USS and pathological size (PS) evaluation have not analysed their systematic differences. The objective ofthis study was to uncover the lack of agreement (bias) between USS and PS measurements.Methods: A retrospective study was performed on 121 patients who had a total or hemithyroidectomy for a solitary nodule. Ordinary least product regression was used to detect anddistinguish constant and proportional bias in unidimensional size measurements between USSand PS evaluation. Three-dimensional volume measurements were compared in a subgroup of31 patients. Pre-specified acceptable limits of interchange were defined as 20% difference.Results: Ordinary least product regression demonstrated no constant or proportional biasbetween the two methods; regression equation: USS = (0.863) + (1.040) × PS. Whennodules were grouped by size, discrepancies between the two methods were observed innodules 10mm but PS