Background: Bilateral papillary thyroid carcinoma (PTC) requires aggressive treatment, such as total thyroidectomy (TT). If there is only an isolated PTC focus in one lobe that can be diagnosed preoperatively, and it is unknown whether there are foci in the contralateral lobe that are too small to be detected, it is difficult to know whether to perform TT or to remove only the lobe with the isolated PTC focus. Here, we investigated the prevalence of and predictive factors for occult bilateral PTC that was only diagnosed unilaterally before surgery. Methods: This retrospective study involved 586 patients with unilateral PTC who were diagnosed preoperatively by ultrasound. They underwent TT and cervical lymph node dissection. According to the pathology, they were divided into unilateral PTC and bilateral (Bil)PTC groups. Student?s t test, chi-squared test, and multivariate analysis were performed to identify features of the malignant tumor that increased the likelihood of malignancy in the contralateral lobe. The prevalence of occult Bil-PTC was calculated. Results: Bil-PTC was found in 70 of 586 (11.95%) PTC patients. Multivariate analysis showed that vascularity (odds ratio[OR]: 2.180, 95% confidence interval [CI]: 1.142-4.162, p = 0.018) and ultrasound diagnosis of lymph node metastasis (USLNM) (OR: 2.056, 95% CI: 1.056-4.004, p = 0.034) were independent predictors of occult Bil-PTC. Conclusion: The prevalence of occult PTC in the contralateral lobe was only 11.95%. Vascularity and USLNM were risk factors for BilPTC. In terms of these preoperative risk factors for PTC, TT should be cautiously performed in patients with preoperative diagnosis of Background: Bilateral papillary thyroid carcinoma (PTC) requires aggressive treatment, such as total thyroidectomy (TT). If there is only an isolated PTC focus in one lobe that can be diagnosed preoperatively, and it is unknown whether there are foci in the contralateral lobe that are too small to be detected, it is difficult to know whether to perform TT or to remove only the lobe with the isolated PTC focus. Here, we investigated the prevalence of and predictive factors for occult bilateral PTC that was only diagnosed unilaterally before surgery. Methods: This retrospective study involved 586 patients with unilateral PTC who were diagnosed preoperatively by ultrasound. They underwent TT and cervical lymph node dissection. According to the pathology, they were divided into unilateral PTC and bilateral (Bil)PTC groups. Student?s t test, chi-squared test, and multivariate analysis were performed to identify features of the malignant tumor that increased the likelihood of malignancy in the contralateral lobe. The prevalence of occult Bil-PTC was calculated. Results: Bil-PTC was found in 70 of 586 (11.95%) PTC patients. Multivariate analysis showed that vascularity (odds ratio[OR]: 2.180, 95% confidence interval [CI]: 1.142-4.162, p = 0.018) and ultrasound diagnosis of lymph node metastasis (USLNM) (OR: 2.056, 95% CI: 1.056-4.004, p = 0.034) were independent predictors of occult Bil-PTC. Conclusion: The prevalence of occult PTC in the contralateral lobe was only 11.95%. Vascularity and USLNM were risk factors for BilPTC. In terms of these preoperative risk factors for PTC, TT should be cautiously performed in patients with preoperative diagnosis of PTC with isolated focus.