HLA class I and II molecules play an important role in the immune system. Thus, alterations in their expression could affect the immunosurveillance against tumors. The expression of these molecules may therefore be one of the factors responsible for oncogenicity, due to their role as restriction elements in T cell recognition. A series of 78 primary laryngeal and 94 breast tumors were evaluated for the expression of MHC class I and II antigens, using monoclonal antibodies. We found a number of cases presenting total loss of HLA-ABC and selective losses of HLA-A and HLA-B antigens. These losses were statistically associated with clinical and pathological parameters such as T stage, degree of differentiation, scores according to the malignancy grading systems and degree of leukocytic infiltration. On the other hand, the de novo expression of DR antigens in laryngeal cancer was found in a group of neoplasms having a peculiar clinical behavior and excellent prognosis. Expression of class II molecules in breast tumors was also related to a better differentiation and prognosis. Thus, HLA class I and II antigen expression is closely related to aggressiveness and prognosis of laryngeal and breast cancer.