To survey the prevalence of asthma in urban and rural residents of Liaoning province in 1999 and evaluate the risk factors for asthma, providing data for research and preventive and therapeutic policies for control of asthma.Stratified cluster disproportional random sampling survey was performed in 116 276 residents by common protocols and questionnaire. Asthma was diagnosed based upon case history, clinical signs and lung function tests. Univariate analysis was employed to evaluate the risk factors for asthma.The overall prevalence of asthma in Liaoning province was 1.25%, the male to female ratio being 1:1.64. The prevalence of asthma in adults was 1.39% and that in children was 0.48%. The onset of asthma occurring in children under the age of 14 accounted for 38.63% of the whole patients. The prevalence was higher in peasants, animal breeders and government officials. The prevalence in the rural area (1.89%) was higher than that in the urban area (0.81%) (P < 0.01). The prevalence in Shenyang city (1.66%) was higher than those in Dalian city (0.88%) (P < 0.01) and Jinzhou (0.71%) (P < 0.01) city. Risk factor analysis showed that 24.7% of the asthmatics had family history; 20.4% had allergic history, the OR of allergic rhinitis being 6.28; 26.0% had smoking history; the OR of cooking oil smoke exposure was 5.94. Asthmatic attack was induced by influenza, cold air, irritant gases and cooking oil smoke in 52.3% approximately 63.3% of the patients. Asthma induced by exercising, fatigue, emotion and allergy was found in more than 41.3% of the patients.Our epidemiologic data reflected the general distribution of asthma in Liaoning province. From the overall prevalence rate, it is estimated that there are about 529 700 asthmatics in the whole province. The data will be helpful for the mass prevention and treatment of asthma.