Pheochromocytomas and paragangliomas are rare neoplasms. Approximately 10% may present malignant behaviour. There are no reliable morphological signs of malignancy, except for the presence of metastasis. We performed morphological and immunohistochemical analysis on 15 pheochromocytomas and 5 paragangliomas aiming to find correlations between the morphological features of the tumours, immunohistochemical expressions of Ki-67 and Bcl-2, and the biological behaviour of the tumours. According to the biological behaviour of the tumors, the patients were divided into an indolent disease group (ID), and an aggressive disease group (AD). The morphological analysis included the PASS core parameters, greatest tumour diameter and weight, as well as age and gender of the patients, survival and disease-free periods after operation. According to histomorphological parameters, tumours were divided into tumours with "benign-like" morphology and tumours with "malignant-like" appearance. The disease course was neither correlated to the PASS score, nor to the individual parameters comprising it. The rest of the morphological parameters and the immunohistochemical expressions of Ki-67 and Bcl-2 were not able to predict the disease course, although we found significantly higher Ki-67 expression in paragangliomas in comparison to pheochromocytomas (p<0.01). Some of the PASS parameters (vascular invasion and presence of atypical mitoses) were positively correlated to the tumour weight (R=0.75; p<0.01, and R=0.56; p<0.05, respectively). The disease course was in positive correlation to the tumour weight, presence of vascular invasion and atypical mitoses; however there were no statistically significant differences regarding those parameters between the ID and AD groups (p>0.05).