1. Hair samples were obtained from subjects at the end of a 6 month smoking-cessation programme carried out with (n = 16) or without (n = 10) the aid of nicotine chewing gum. The axial distribution of nicotine along the hair shaft was determined and compared both with the self-report of smoking behavior and with the concentration of carbon monoxide (CO) in expired air measured at 1 month intervals. 2. A gradual decrease in nicotine content along the hair shaft corresponded to the decrease in self-reported number of cigarettes smoked daily in both the nicotine chewing-gum treated and untreated subjects and with the clinical assessment of abstinence. 3. There was reasonable agreement between the CO content of expired air, the self-reported number of cigarettes smoked daily and the axial distribution of nicotine in 22 of 26 subjects. In the other four subjects there was an apparent disagreement between the CO content of expired air and the other parameters. 4. This study showed that, as a routine marker of smoking status, hair analysis may be preferable to repeated CO measurements since only a single sample at the end of an abstinence programme might be sufficient. However, one problem is that nicotine may dissociate slowly from melanin to limit the ability to mark a sudden cessation of smoking. Also, some nicotine in hair might be attributable to nicotine adsorbed to the outside of hair.