Objective To determine the expression of CD34 (to evaluate vascularity) and Ki-67 (as a proliferative index) in a variety of phaeochromocytomas and thus assess the utility of these markers as possible predictors of malignancy. Patients and methods Deparaf®nized tissue sections from 21 patients with benign and four with malignant phaeochromocytoma were assessed immunohistochemically to evaluate microvascular counts (using CD34 antigen) and the proliferative index (Ki-67 antigen, monoclonal antibody MIB-1).Results There was no statistical association between high microvascular counts and malignancy, but there was between the MIB-1 score and malignancy (P<0.001). Conclusions A high MIB-1 score in phaeochromocytoma suggests malignant behaviour in the component cells.The MIB-1 score may be a useful indicator for diagnosing malignant phaeochromocytoma.