['Nitric oxide (NO), a gas produced by cells lining the respiratory tract, has been reported to be decreased in the exhaled air of cigarette smokers. We hypothesized that smoking cessation would result in an increase in exhaled NO.', 'Comparison of exhaled NO measured from nonsmokers, cigarette smokers, and smokers after smoking cessation.', 'University outpatient smoking cessation clinic.', 'Twenty-five cigarette smokers and 23 normal, nonsmokers.', 'Exhaled NO was measured by three techniques: (1) a peak oral method; (2) a mean oral method; and (3) a nasal method. The smokers were given nicotine patches and instructed to return after 1 and 8 weeks. The exhaled NO determinations were repeated on each visit.', 'Compared with nonsmokers, smokers had decreased NO levels measured by all three methods (p<0.05, each comparison). Nineteen smokers returned after 1 week. Fourteen were successfully abstinent from cigarettes and their exhaled NO increased compared with baseline (p<0.01 for each method) but not in the five subjects who had not successfully quit smoking (p>0.05 for each method). Ten subjects returned after 8 weeks. The exhaled NO levels increased further and were not significantly different from the normal nonsmokers for the peak oral and nasal NO methods (p>0.2), but were still lower than the normal nonsmoker mean oral NO (p=0.018).'] Is smoking cessation associated with an increase in exhaled nitric oxide?