In asthma self-management training, often self-treatment guidelines are included, because increased knowledge of asthma alone is not sufficient to change behaviour. One way to achieve behavioural changes is by increasing the patient's general and asthma-specific self-efficacy expectancies. This refers to beliefs in one's capabilities to execute the recommended course of action successfully. We wanted to assess whether high generalised and asthma-specific self-efficacy expectancies were predictive of adequate self-management and self-treatment behaviour. A questionnaire was sent to 4563 persons (18-65 years) who had used inhaled medication in 1993. Self-management and self-treatment behaviour were operationalised through a hypothetical scenario of a slow-onset asthma exacerbation. Of all 1262 asthmatic patients, 39.3% showed adequate self-treatment behaviour (self-adjusting their inhaled or oral steroids when appropriate). Age, asthma-specific outcome expectancies and knowledge were predictive of adequate self-treatment. Adequate self-management behaviour (self-treatment or seeking medical help) was observed in 56.4% of patients. Intentions towards self-management and asthma-specific knowledge were significant. Only knowledge has a relevant influence on both. Asthma-specific knowledge is the only factor that seems relevant for adequate self-management and self-treatment behaviour, which might be explained by the hypothetical nature of the scenario. When patients experience a real asthma exacerbation, self-efficacy expectancies will become more important. Only if knowledge of what to do is present will patients be able to show proper self-management and self-treatment behaviour. Our results suggest that self-treatment guidelines are only effective in combination with patient education, which is important for optimal control of their disease.