It had been observed that some cystic nodules change morphologically with ultrasonographic (US) features suspicious for malignancy. The aim of this study was to evaluate the US characteristics of benign cystic nodules mimicking papillary thyroid carcinoma (PTC) during interval changes. Between January 2009 and October 2009, 26 patients with benign cystic nodules showing marked hypoechogenicity in US during the follow-up period were enrolled. During the same period, 38 patients with marked hypoechogenicity in US were enrolled for the PTC group. We evaluated the differences in US characteristics between the 2 groups. Nodule size, margin, echogenic dot and vascularity were not significantly different between the 2 groups. Nodule shape was significantly different between the 2 groups with a lower prevalence of taller than wide in the benign cystic group (11.5% vs. 39.5%, P=0.022). Other coexisting cystic nodules were more frequently observed in benign cystic group (48.3% vs. 5.3%, P=0.001). If echogenic dot was detected in benign cystic nodule, it was more than 1 mm in size without posterior acoustic shadowing unlike echogenic dots in the PTC group. In conclusion, some of the benign cystic nodules may have suspicious malignant features on US during interval changes. A careful assessment of US findings and a previous history may be of value in discriminating them from PTC.