Ninety-five patients with papillary thyroid carcinoma (PTC) who received primary surgical treatment in 1983 at Kuma Hospital and were followed until 1992 were the subjects of this study. Initial therapy was tumor resection for 5 patients, lobectomy for 23 patients, total thyroidectomy with unilateral modified neck dissection for 60 patients, and total thyroidectomy with bilateral modified neck dissection for 7 patients. Clinical stage at diagnosis was as follows. Class I included 28 patients with intrathyroidal disease, class II included 60 patients with positive cervical lymph nodes, and class II included 7 patients with tumor invasion into tissue outside of the thyroid gland. Recurrence of the tumor was evaluated according to lymphocytic infiltration in the thyroid gland. Group A consisted of 36 patients with PTC associated with lymphocytic infiltration, 26 with infiltration surrounding the tumor, 3 with infiltration inside of the tumor, and 7 with both. Group B consisted of the remaining 59 patients with PTC with no lymphocytic infiltration. There were no differences in age, sex, initial tumor size, or initial treatment between groups A and B. Antithyroglobulin antibody and/or antimicrosomal antibody were positive in 16 patients from group A and 4 patients from group B (P < 0.001). Class I included 14 patients from each group, class II included 22 patients from group A and 38 patients from group B, and class III included 7 patients, all from group B. Recurrence of the tumor was found in only 1 group A patient (2.8%), but in 11 patients of group B (18.6%). The percentage of patients free from recurrence over the 10 yr of follow-up in group A was significantly higher than that in group B (by Cox-Mantel test, P < 0.01). The time between initial treatment and recurrence was 2-10 yr. In comparing the clinical stage at the time of initial treatment, recurrence was found in 1 class II patient from group A (4.5%) and in 1 class I (7.1%), 6 class II (15.8%), and 4 class III (57.1%) patients from group B. No patients died during the 10 yr of follow-up. In conclusion, 1) lymphocytic infiltration surrounding the tumor or inside the tumor in PTC might be of use as a means for predicting a favorable prognosis; and 2) class II or class III patients with no lymphocytic infiltration had a high rate of recurrence.