INTRODUCTION: In France about 3.5 million people suffer from asthma, with an estimated annual cost of 1.5 billion euros. Few data on the components of the cost are available. To develop a medico-economic model to identify the mean annual cost of a patient with asthma together with the components of the cost and the consequences of different modalities of management (out-patient and in-patient). METHODS: A first degree homogeneous Markov model was constructed, based on published data, to identify the different states of health of asthmatic patients, their development and probabilities of change from one to another. RESULTS: The mean annual cost of asthma is 631 +/- 299 euros; 298 euros for stable patients, 1,052 for those who have at least one acute attack and 3,811 for those hospitalised. Actions leading to better control of the disease in the community (prevention and management of attacks) would produce a larger reduction in mean costs than actions undertaken in patients seen as an emergency or admitted to hospital. CONCLUSIONS: From the point of view of society better control of asthma in the community is the most effective strategy to reduce expenditure.