['To clarify whether or not the rate of tumour growth is a useful measure for distinguishing papillary thyroid cancer from benign nodules.', 'Retrospective study.', 'Teaching hospital, Japan.', 'Fourteen patients with 27 nodules (<20 mm) followed up for 20 months or more.', 'High resolution ultrasonography (US).', 'Growth rate.', 'There were 15 papillary thyroid cancers and 12 benign nodules. The median tumour diameters at the first and the last US examination were 9.0 mm (range 6-19) and 9.0 mm (range 5-21) in papillary thyroid cancers followed up for 37 months (range 21-85), and 11.5 mm (range 4-19) and 11.5 mm (range 5-23) in benign nodules followed up for 38 months (range 20-84), respectively.'] Does the rate of tumour growth distinguish between malignant and benign thyroid nodules?