Cancer-associated O-glycopeptide combination epitopes derived from the VNTR of MUC1 are disclosed. Autoantibodies present in human sera target the combination epitopes and are reduced or absent in cancer patients. The epitopes are useful as therapeutic and immunoprophylactic cancer vaccines. Monoclonal antibodies directed against the epitopes are also useful as immunotherapeutics for treatment and prevention of cancer. Diagnostic methods using the epitopes and antibodies are also disclosed.