Surgical materials obtained from 5 normal human thyroids and 19 diseased tissues (4 with Grave's disease, 4 adenomatous goiters, 3 follicular adenomas, 2 papillary carcinomas, 4 follicular carcinomas and 2 undifferentiated carcinomas) were investigated immunohistochemically using the avidin-biotin-peroxidase complex technique with anti-proliferating cell nuclear antigen (PCNA) antibody and epidermal growth factor receptor (EGFR) antibody. The results of this study suggested the following: (1) Labeling indexes of PCNA positive cell nuclei were shown in high grade malignant cases accompanied by capsular invasion and lymph node metastasis, in undifferentiated carcinoma cells, and in proliferative lesions of benign diseases. These results suggest that PCNA is indicative of proliferative ability in thyroid diseases. (2) On thyroid tumors, EGFR regions with strong positive staining showed many PCNA positive cell nuclei. EGF appears to have a significant association with proliferative ability in thyroid diseases.