Thyrocyte HLA-DR expression in association with lymphocytic infiltration has been demonstrated in autoimmune diseases of the thyroid. HLA-DR expression has been noted both in association with lymphocytic infiltration and in its absence. However, no prognostic significance was attributed to these findings. The purpose of this study on papillary carcinoma of the thyroid was to examine how tumour behaviour relates independently to HLA-DR expression and lymphocytic infiltration. Fifty patients with papillary carcinoma with known metastatic status were examined for HLA-DR expression utilizing the immunoperoxidase conjugated indirect method. The patients were divided into three groups: no metastases (n = 16), regional lymph node metastases only (n = 25), and distant haematogenous metastases (n = 9). Overall HLA-DR expression was noted in 6, 36 and 55% of the respective groups. Tumour HLA-DR expression without lymphocytic infiltration was noted in 0, 4 and 33% of the respective groups. Tumour HLA-DR expression with lymphocytic infiltration occurred in 6, 32 and 22% of the respective groups. From these findings we conclude that HLA-DR expression with lymphocytic infiltration occurs more frequently than HLA-DR expression in the absence of infiltration. The concurrence of HLA-DR expression and lymphocytic infiltration is most commonly associated with regional lymph node metastasis. HLA-DR expression in the absence of lymphocytic infiltration occurs in tumours with distant haematogenous metastases and is an uncommon feature of tumours that have not metastasized or have metastasized only to lymph nodes. Tumour HLA-DR expression in the absence of lymphocytic infiltration may indicate the potential for more aggressive behaviour.