The relationship of environmental tobacco smoke to lung cancer risk in lifelong non‐smokers is commonly studied using marriage to a smoker as the index of exposure. As smokers tend to marry smokers, relative risk estimates will be biased if some current or former smokers are misclassified as lifelong non‐smokers. This paper shows how various factors affect the magnitude of the bias and describes a method for obtaining misclassification‐adjusted relative risk estimates. Application of the method to U.S. and Asian data for women suggests misclassification is an important determinant of the slight excess risk observed in non‐smokers married to smokers. Reasons why our conclusions differ from those of others are discussed, as are other difficulties in interpreting the association between spousal smoking and lung cancer risk.