Asthma, a disease of the respiratory system, is the most common chronic medical condition among children. School-aged children with asthma evidence increased absenteeism, decreased school performance, restriction of various activities, poor peer relationships, and emotional and behavioral difficulties. Asthma can be triggered and exacerbated by emotions and stress, and therefore, it has been treated with psychological interventions. Relaxation and guided imagery (RGI), an intervention based on the theoretical position that the mind and body interact in determining health, has been shown to be effective in improving mental and physical health outcomes with a myriad of medical conditions, yet it has not been adequately studied with childhood asthma. This investigation analyzed the effect of RGI on lung function [forced expiratory volume in 1 second (FEV1) and forced expiratory flow 25–75 (FEF25–75)] and anxiety by employing a multiple baseline design across four middle school students with asthma. With the introduction of the intervention, it was found that FEV1 improved and anxiety decreased in all students. FEF25–75 improved in three of the four participants. The effect sizes for the four participants ranged from −0.98 to −1.88 for FEV1, 0.20 to −1.93 for FEF25–75, and 2.19 to 4.06 for anxiety. © 2003 Wiley Periodicals, Inc. Psychol Schs 40: 657–675, 2003.