The APUD concept has postulated that pulmonary carcinoids and small cell carcinomas arise from the neural crest. In development from hypothesis to tautology is traced, and evidence is presented that all pulmonary epithelial tumors arise from the primitive endoderm. Morphologic studies show that a dynamic spectrum exists. Not only do various cell types appear within a single section, but cell types may change from biopsy to autopsy with or without chemotherapy. The spectrum is sustained at the ultramicroscopic level in regard to organelles such as desmosomes, tonofibrils, and dense core granules. Secretory products such as ACTH and L-dopa decarboxylase also show that all lung cancers are related. Epidemiologic evidence indicates that small cell carcinomas in uranium miners occur after prolonged squamous cell dysplasia, and that carcinoids occur independently of external carcinogens, but show transitions to other tumors. Finally, experimental evidence indicates that the K cells, to which carcinoids are most closely related, are of local origin.