We review the significance of a network of proneural basic helix-loop-helix (bHLH) factors. Immunohistochemically, pulmonary neuroendocrine cells (PNECs) are positive for Mash1, one of the activator bHLHs, and non-PNECs such as Clara cells are positive for Hes1, one of the repressor bHLHs. Since mice deficient for the Mash1 gene do not possess PNEC and mice deficient for the Hes1 gene have many PNECs, it is suggested that a network of bHLHs work in cell fate determination of lung epithelium. Moreover, the Notch pathway could play a role in cell differentiation mechanisms in the lung because this signaling pathway has been reported to work in various tissues. PNECs have been reported to modulate various nonneoplastic human lung diseases. We demonstrate that PNECs in usual interstitial pneumonia and hASH1 (human homolog of Mash1) are upregulated in diseased lung tissues. Moreover, studies of small cell carcinoma and non- small cell carcinoma suggest that neuroendocrine differentiation could be regulated by hASH1. In non-small cell carcinoma, Hes1 and Notch signaling may have roles in maintaining cell differentiation. Thus, a network of bHLHs and Notch signaling are important in cell differentiation of normal and pathologic lung epithelial cells.