This study examines the relationship among psychosocial factors, asthma symptoms, and peak expiratory flow rate (PEFR) in the natural environment. Twenty adult asthmatics wore preprogrammed wristwatches that prompted them to assess PEFR, asthma symptoms, and psychosocial factors five times a day for 10 days. Psychosocial variables (activities, locations, social contacts, mood, and stressors) were strongly related to PEFR and asthma symptoms, suggesting that they may play a more important role in disease expression than has been previously thought. Diurnal cycles of asthma symptoms and PEFR were observed. However, statistically controlling for psychosocial factors eliminated diurnal cycles for PEFR or asthma symptoms, indicating that psychosocial factors are a major contributor to the observed diurnal cycle in PEFR and symptoms. These relationships underscore the need to include psychosocial factors in future asthma research.