The invention relates to the field of antibodies. In particular it relates to the field of therapeutic (human) antibodies for the treatment of ErbB-2/ErbB-3 positive tumor. More in particular it relates to treating tumors with a high ErbB-2/ErbB-3 cell-surface receptor ratio. Also encompassed are methods for treating patients not previously treated with an ErbB-2 specific therapy or with an ErbB-3 specific therapy.