Ernst Wynder did not believe that environmental tobacco smoke (ETS), also known as secondhand smoke, was a cause of lung cancer because his own data did not support this. His view on the issue may have begun to change with our initial studies, carried out at the American Health Foundation, showing that metabolites of the tobacco-specific lung carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) were present in the urine of non-smokers exposed to ETS. The metabolites - 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) and its glucuronides (NNAL-Glucs) - have now emerged as leading biomarkers for the study of carcinogen exposure in non-smokers exposed to ETS. Subsequent work has consistently demonstrated elevated levels of NNAL plus NNAL-Glucs in the urine of people exposed to ETS in various field studies and throughout life. These studies strongly support the epidemiologic data demonstrating that ETS exposure causes lung cancer in non-smokers and have likely had an impact on tobacco control.