Typical and atypical carcinoids constitute less than 5% of lung tumors. They and small cell undifferentiated lung cancers (SCLC) belong to the same family of apudomas arising from bronchopulmonary Kulchitzky cells. To reflect the overlap among these cancers, the authors suggest calling them Kulchitzky cell carcinomas (KCC); to indicate their spectrum of aggressiveness, the authors suggest calling typical carcinoids KCC-I, atypical carcinoids KCC-II, and small cell cancers KCC-III. One hundred fifty-six KCCs were reviewed: 115 were KCC-I and 41 were KCC-II. The ratio of women to men equals 2:1. At time of initial diagnosis, all patients with KCC-I, except one patient, were in Stage I. Among patients with KCC-II, 16 (39%) were in Stages II or III at time of presentation. The incidence of carcinoid syndrome was 1.9%. Treatment was lobectomy in 112 (72%) of patients, the remainder having lesser resections or pneumonectomy in approximately equal distribution. Our data cannot support the use of radical resection in the treatment of KCC because none of the patients died of local recurrence. The mean diameters of KCC-I and -II tumors were 1.5 and 2.8 cm, respectively. Increased mitotic activity and tumor necrosis were reliable criteria for diagnosing KCC-II. Electron microscopic examination did not help in differentiating KCC-I and KCC-II. Thorough sampling of the entire tumor was found to be mandatory for precise diagnosis and for differentiation from KCC-III (SCLC). Measurement of nuclear DNA was done using integrated optical density (IOD) by image analysis. The IODs of KCC-I, -II and -III were 1.36, 1.55 and 1.94, respectively. These significant differences (P less than 0.001) correlated with the aggressiveness of the cancers. Of patients with KCC-I, 1.7% succumbed to KCC; this included one patient reported to have died of KCC-III (SCLC). Of 41 patients with KCC-II, 11 (27%) died of KCC; this includes at least 3 deaths from KCC-III.