In recent years, incidental thyroid tumors detected by imaging studies such as ultrasonography, CT, MR, and PET are increasing in number. It is necessary to perform ultrasonography and fine-needle aspiration cytology in order to diagnose thyroid tumors. The most important aspect of ultrasonic diagnosis is to make the differential diagnosis of malignant tumors from benign tumors. However, fine-needle aspiration cytology is generally regarded to be unnecessary in cases of small and benign tumors detected on ultrasonography. It is important to detect malignant tumors, especially papillary carcinomas, on ultrasonography. Most papillary carcinomas present typical and malignant findings on ultrasonography. Papillary microcarcinomas (≦10 mm) are classified as either high risk or low risk by ultrasonography in our hospital. The result of observation without immediate surgery (or “active surveillance”) for patients with papillary microcarcinomas in the low-risk group proved to be satisfactory in terms of clinical management. Most patients with benign tumors should be observed without surgery during clinical management. Physicians at general hospitals must decide at what point during clinical management to refer patients with thyroid tumors to a facility specializing in thyroid diseases.