['Bronchopulmonary carcinoid tumors can be divided into typical and atypical carcinoids according to their histologic and clinical features. Atypical carcinoids tend to have less predictable biologic behavior and are associated with a worse prognosis than typical carcinoids. The authors examined apoptosis and the expression of apoptosis regulating proteins Bcl-2, Bax, Bak, and Mcl-1 in 21 typical and 10 atypical bronchopulmonary carcinoid tumors. To find features distinguishing the growth of these types of tumors, the expression of p53 protein and the proliferation marker Ki-67 were also investigated. Finally, the parameters were compared with clinical follow-up data.', "Thirty-one bronchopulmonary carcinoid tumors were examined by using in situ 3'-end labeling of DNA (TUNEL) for apoptosis and immunohistochemical staining methods for Bcl-2, Bax, Bak, Mcl-1, p53, and Ki-67 in formalin fixed, paraffin embedded tissue specimens.", 'The apoptotic indices (AIs) were low in general (mean 0.18%), but higher AIs (> 0.5%) were observed significantly more often in atypical than in typical carcinoids (P < 0.008) in association with shortened survival (P < 0. 008). No relation between apoptosis and Bcl-2, Bax, Bak, or Mcl-1 expression was found. Expression of p53 protein was detected in only three atypical carcinoids, which also had significantly higher AIs than p53 negative tumors (P < 0.03). Proliferation rates measured by Ki-67 index were unusually low; the highest proliferation rates were observed in atypical carcinoids. There was a positive correlation between AI and Ki-67 indices (P < 0.01).'] Do atypical bronchopulmonary carcinoids show a tendency toward increased apoptotic and proliferative activity?