A distinctive feature of malignant adrenocortical neoplasms is decreased major histocompatibility complex (MHC) class II molecule expression. However, it is unknown whether there exists a restriction to certain MHC genotypes and whether this involves alterations of the Fas/Fas ligand system and thereby affects tissue homeostasis. Therefore, MHC class II phenotype and genotype and expression patterns of the Fas/Fas ligand system were investigated in 24 adrenocortical tumors (n(Adenomas) = 14, n(Carcinomas) = 10) and an adrenal cancer cell line (NCI-H295). No MHC class II antigen expression was detected in carcinomas. The DRB1*01 genotype was found in 54.5% of patients with carcinoma (P = 0.046). No prevalence of any genotype could be detected in patients with adenomas, which exhibited varying levels of antigen expression. Fas receptor expression was 75.0% in adenomas compared with 20.0% in carcinomas (P = 0.0196), whereas ligand expression was 37.7% in adenomas and reached almost 100% in the carcinomas investigated in this study (P = 0.0033). In summary, the DRB1*01 genotype may be correlated to a higher risk for malignancy. Additional studies on MHC class II genotype and phenotype and the altered Fas/Fas ligand system in adrenal neoplasms may help to identify mechanisms of immune escape and suggest new diagnostic approaches.