The thyroids from 101 consecutive autopsies from Finland were subserially sectioned at 2- to 3-mm intervals. From 36 thyroids, 52 foci of occult papillary carcinoma (OPC) were found, giving a prevalence rate of 35.6%, the highest reported rate in the world. The rate was higher, although not significantly, in males (43.3%) than in females (27.1%), but it did not correlate to the age of the patients. Twenty-six glands contained one tumor focus and ten glands contained two to five tumor foci. Only a minority of the smallest tumors can be detected with the method used. The probable number of OPCs over 0.15 mm in diameter was calculated to be about 300 in this material. The tumor diameter varied from 0.15 mm to 14.0 mm, with 67% of tumors under 1.0 mm. The smallest tumors were usually circumscribed and were composed almost solely of follicles. Larger tumors had more papillary structures and were often invasive. Fibrosis and, in the largest OPCs, lymphocytic reaction were seen around the invasive islands. All tumors were positively stained for thyroglobulin and all but one of the tumors stained positively for epidermal keratin. OPC appears to arise from follicular cells of normal follicles. Apparently the great majority of the tumors remain small and circumscribed and even from those few tumors that grow larger and become invasive OPCs only a minimal proportion will ever become a clinical carcinoma. According to the study, OPC can be regarded as a normal finding which should not be treated when incidentally found. In order to avoid unnecessary operations it is suggested that incidentally found small OPCs (less than 5 mm in diameter) were called occult papillary tumor instead of carcinoma.