The recent prevalence of ultrasound-guided fine-needle aspiration biopsy has resulted in a marked increase in the number of patients with papillary microcarcinoma (maximum diameter, </= 10 mm) of the thyroid detected by this sophisticated tool. On the other hand, it is debatable whether patients with papillary microcarcinoma should always undergo surgery after diagnosis, because a high incidence of occult papillary carcinoma has been observed in autopsy studies. Thus, we proposed observation without surgical therapy as a treatment option in 732 patients diagnosed with papillary microcarcinoma by the above technique from 1993 to 2001. One hundred sixty-two patients chose observation and were classified as the observation group. During the follow-up period for patients in the observation group, more than 70% of tumors either did not change or decreased in size compared to their initial size at diagnosis. They enlarged by more than 10 mm in 10.2%, and lymph node metastasis in the lateral compartments appeared in only 1.2% of patients during follow-up. On the other hand, 570 patients chose surgical treatment at diagnosis and 56 patients in the observation group who underwent surgery after a period of follow-up were classified as the surgical treatment group. Of these 626 patients, lymph node dissection was performed in 594 patients, and metastasis was confirmed histologically in 50.5%. Multiple tumor formation was seen in 42.8% of patients. In this group, the rate of recurrence was 2.7% at 5 years and 5.0% at 8 years after surgery. Our preliminary data suggest that papillary microcarcinomas do not frequently become clinically apparent, and that patients can choose observation while their tumors are not progressing, although they are pathologically multifocal and involve lymph nodes in high incidence.