One hundred and sixty-two smokers who participated in an aggressive smoking cessation program underwent analysis of expired air carbon monoxide (CO) and serum thiocyanate (SCN) at entry and one year later. Persons who failed to quit smoking had higher baseline CO and SCN and smoked more cigarettes per day than did those who succeeded in quitting. The use of CO, SCN, number of cigarettes smoked, age, and blood pressure at baseline permitted accurate classification of over 70 per cent of the subjects into groups which would succeed and fail in their effort to quit smoking. Persons who failed to quit reduced the reported number of cigaretts smoked per day by one-third, but there was no corresponding reduction in CO and SCN, suggesting that self-reporting of a reduction in the number of cigarettes smoked may not lead to a corresponding decline in exposure.