The present study examined the effect of smoking reduction and cessation on asthma regulation and biomarkers of exposure to cigarette smoke. In a prospective open design, we allocated 220 asthmatics among three groups: (a) Smoking reduction (reducers), with the aim of smoking fewer than seven cigarettes per day, (b) complete smoking cessation (abstainers), or (c) continuation of usual smoking (continuing smokers). Subjects used nicotine chewing gum or an oral nicotine inhaler to promote reduction and cessation. We monitored changes in the biomarkers carbon monoxide, cotinine, and thiocyanate, and in peak flow, medicine use, bronchial reactivity, and asthma symptoms. The analysis used the three outcome groups, regardless of original allocation to treatment groups. At 4 months, analysis of abstainers (n = 27), reducers (n = 33), and continuing smokers (n = 50) showed marked, statistically significant decreases in expired carbon monoxide of 17 ppm (abstainers) and 15 ppm (reducers); in plasma cotinine of 124 ng/ml (abstainers) and 122 ng/ml (reducers); and in plasma thiocyanate of 5.03 ng/ml (abstainers) and 3.74 ng/m (reducers). For abstainers, we observed improvements in the asthma-specific quality-of-life score, and reductions in self-reported day and night use of rescue beta2-agonists, in doses of inhaled corticosteroids, in daytime asthma symptoms, and in bronchial hyperreactivity. For reducers, smaller improvements occurred for night use of rescue beta2-agonists, doses of inhaled corticosteroids, and bronchial hyperreactivity. Smoking cessation resulted in a marked decrease in three biomarkers of cigarette smoke inhalation and improved asthma regulation, whereas smoking reduction had a less pronounced effect on biomarkers and only a small effect on asthma regulation.