We evaluated effects of smoking cessation on lung function and respiratory symptoms of residents 25 to 64 yr of age from 3 communities in the Los Angeles area who completed a detailed respiratory questionnaire and measurements of forced expired volumes and flow rates, closing volume, and closing capacity at 2 times (T1 and T2) 5 yr apart. Results were analyzed in 2,401 participants who fit into 4 smoking categories: never smokers (414 males, 737 females); former smokers (294 males; 172 females); quitters between T1 and T2 (106 males, 62 females); and continuing smokers (278 males, 338 females). Covariance analysis was used to determine differences in lung function across smoking categories at T1 and T2 (adjusted for T1 values) and differences in decline in lung function between T1 and T2. Chi-square analysis was used to compare continuing smokers and quitters with respect to changes in respiratory symptoms. In this population, smoking at T1 was associated with impairment in all indexes of lung function evaluated. Smoking cessation led to significant improvement in symptoms of cough, wheeze, and phlegm production, and to significantly less decline in indexes of small airway function during 5 yr compared with measurements in continuing smokers. However, at T2, lung function still was lower among quitters compared with former and never smokers. Forced expiratory volume in one second (FEV1) was marginally improved in women who quit compared with those who continued to smoke.(ABSTRACT TRUNCATED AT 250 WORDS)