Fifty-nine cases of tumors, 51 taken from the thyroid gland including 17 well-differentiated papillary, 1 medullary and 3 follicular carcinomas, 14 follicular adenomas, 16 nodular goiters and 8 cases selected from other sites; 5 breast carcinomas; 1 melanoma; 1 Merkel cell tumor; and 1 squamous cell carcinoma of the uterine cervix, were investigated for intercellular adhesion molecule-1 (ICAM-1) expression. All cases of well-differentiated papillary thyroid carcinoma showed positivity for ICAM-1, whereas the follicular carcinomas, follicular adenomas, and all but one nodular goiters were negative. It is suggested that the occurrence of ICAM-1 on the thyroid cell surfaces in well-differentiated papillary thyroid carcinomas may contribute to the understanding of their biology and could be of potential significance for diagnostic purposes.