The relationship between stress and bronchial asthma has been the subject matter of many studies. A review of empirical results suggests that stress might affect patients suffering from asthma. However, this relation seems to be idiosyncratic, rather than a generalised phenomenon. The present paper studies individual differences in asthma reactivity to stress and the psychological and clinical variables related to such reactivity. 90 adult asthmatics were recruited from an outpatient allergy health service. Clinical and psychological variables, such as severity and type of asthma, coping style and social support were assessed. Additionally, subjects filled out daily self-records with data concerning pulmonary function, number of asthmatic attacks, intensity of asthmatic symptoms, and degree of stress during a period of four weeks. Results showed that patients’ vulnerability to the effect of daily stress varies along a continuum. Approximately 45% of the sample showed a significant and negative relationship between daily stress and asthma. Moreover, the frequency with which emotions trigger off asthma attacks in each subject, the type of asthma, and the coping skills of the subjects are significantly related to the direction and the intensity of the relationship found between stress and asthma. Therefore, these variables seem to be central in determining which asthmatics could benefit from participating in interventions directed to reduce stress as a complement to traditional treatments.