Human leukocyte antigen (HLA) classes I and II molecules are essential for antigen presentation to cytotoxic T cells and helper T cells, respectively. Consequently, they may play a role in anticancer immunotherapy as well. We studied whether the pretreatment HLA phenotype of the tumor is predictive for response to interferon immunotherapy in vivo. Therefore, renal cell carcinoma (RCC) primary tumor lesions from 31 patients treated with interferon-alpha and interferon-gamma (13 responders and 18 nonresponders) were analyzed retrospectively for HLA antigen expression with immunohistochemical methods. Furthermore, from eight patients, pretreatment metastatic lesions were examined. In the primary tumors HLA class I expression was high: in 26 of 30 lesions more than 50% cells were stained. HLA class II expression was mostly low: in 14 of 31 primary tumors less than 5% cells were stained. A significant correlation was found between HLA phenotype of primary tumors and corresponding metastases. There was no association between tumor HLA classes I and II antigen expression and clinical response to interferon therapy. In conclusion, pretreatment HLA phenotype of RCC has no predictive value for outcome of interferon immunotherapy. A role for treatment-induced changes in HLA expression in vivo, however, can not be excluded. These findings do not provide indications for the working mechanism of interferon immunotherapy in vivo.