BACKGROUND Asthma and rhinitis are often comorbid conditions, and several studies have suggested that rhinitis often precedes asthma. Sensitization to allergen has been shown to be one of the strongest determinants of incident asthma, but little is known about the effects of cigarette smoking among individuals with allergic rhinitis. OBJECTIVE We sought to evaluate the importance of cigarette smoking as an additional risk factor for incident asthma in a cohort of hospital-referred nonasthmatic adult subjects with allergic rhinitis. METHODS The study population selected at baseline was invited for a follow-up visit 10 years later to check for possible asthma features. Categories of smokers, exsmokers, and never smokers were used in the analyses together with pack-years to calculate the level of cumulative exposure. RESULTS Complete data were available from 325 patients. Smoking was significantly related to the risk of incident asthma, with the odds ratio (OR) being 2.67 (95% CI, 1.70-4.19) for univariate and 2.98 (95% CI, 1.81-4.92) for multivariate analyses. A clear dose-response association for exposure to tobacco and risk of new-onset asthma was observed in the multivariate analyses: those with 1 to 10 pack-years had an OR of 2.05 (95% CI, 0.99-4.27), those with 11 to 20 pack years had an OR of 3.71 (95% CI, 1.77-7.78), and those with 21 or more pack-years had an OR of 5.05 (95% CI, 1.93-13.20) compared with never smokers. CONCLUSIONS The current findings support the hypothesis that cigarette smoking is an important independent risk factor for the development of new asthma cases in adults with allergic rhinitis.