Bronchopulmonary well-differentiated neuroendocrine carcinoma (WDNEC) represents a more aggressive neoplasm than does typical carcinoid. Its biological behavior is variable and cannot be predicted on the basis of histopathological features. Nineteen typical carcinoids and 23 WDNECs were studied in order to obtain multiple parameters that should be used in the differential diagnosis between these two lesions and as prognostic markers of WDNEC. Flow-cytometry was performed on paraffin-embedded sections. Mutant p53 protein, the bcl-2 oncoprotein and the Ki-67 antigen were detected by immunohistochemical methods and evaluated quantitatively. WDNEC was more frequently aneuploid than typical carcinoid, had a higher percentage of Ki-67 positive nuclei and presented more frequently the mutant p53 protein. In WDNEC, the mutant p53 (p = 0.001), the bcl-2 oncoprotein (p = 0.002) and the high expression (> or = 16%) of Ki-67 (p = 0.0021) were associated with poor prognosis. The prognostic significance of mutant p53 and bcl-2 oncoprotein could be confirmed by Cox multiple regression survival analysis (p = 0.0005). It seems to be advisable to evaluate these features for the management of the patients affected by WDNEC.