Researchers have recently shown that small cell lung cancer (SCLC) cell lines secrete acetylcholine (ACh) into tissue culture medium. Treatment of the cell lines in vitro with either muscarinic (atropine) or nicotinic (mecalmylamine) antagonists reduced cell growth in cell lines that produced high quantities of ACh. This effect of ACh inhibition was seen with both inhibitors but not with cell lines that had a low baseline level of ACh secretion. Previous data from the same group (Cancer Res 2003;63:214–2) found that 13 of 26 archival SCLC expressed the biosynthetic enzyme for ACh production, choline acetyltransferase. These studies show that SCLC can synthesise and secrete ACh, and that released non-neuronal ACh stimulates SCLC cell growth. Identification of this autocrine loop provides a potential new avenue for therapeutic intervention in SCLC cell lines that are high ACh secretors. These data also reinforce the importance of smoking cessation, given the potential of exogenous nicotine to promote tumour growth.