Morphological criteria for malignant pheochromocytoma remain to be developed According to the WHO recommendations, the sole absolute criteria is the presence of metastases in the organs normally containing no chromaffin tissue. Such signs as cellular and nuclear polymorphism, mytotic activity, vascular invasion, capsular ingrowth are not sufficient to describe a pheochromocytoma as malignant. It is equally dfficult to differentiate between malignant and benign tumours based on histological data since histologically mature neoplasms can produce metastases. Based on the results of original studies, the authors believe that such histological features as vascular and capsular invasion do not necessarily suggest unfavourable prognosis. Therefore, the conclusion of malignancy based on such features can not be regarded as absolute. Probably such neoplasms should be called "pheochromocytomas with morphological signs of malignant growths". They should be referred to the tumours with uncertain malignancy potential based on the known discrepancy between morphological structure and biological activity of neoplasms. Comparative studies of clinical and morphological features of pheochromocytomas showed that their histological type (alveolar; solid, dyscomplexed, trabecular) and morphological signs of malignant growth influence both the clinical picture and arterial hypertension. There are no significant relationship between the above morphological signs, timour mass and clinical manifestations of pheochromocytomas.