The reliability of self-reporting on smoking habits has been evaluated by comparing the reported smoking habits with the concentration of serum thiocyanate, which is higher in smokers than in non-smokers and increases with increasing cigarette consumption. When a smoker stops smoking, the level of serum thiocyanate decreases and falls to the level observed in non-smokers after about one month. When the questions asked about smoking were neutral, the reported smoking habits were generally reliable. However, in cases where the questions asked and the interview situation could be regarded as unpleasant, the reported smoking habits were not always correct. Some smokers underreported the number of cigarettes they smoked on a daily basis and some of them denied smoking altogether.