Historically, asthma has been described in functional terms as reversible airways obstruction and bronchial hyperresponsiveness. Whilst there is no agreed definition of asthma, studies involving bronchoalveolar lavage and endobronchial biopsy are highlighting inflammatory processes as an essential component of the disorder. With the success already achieved in quantifying mast cells, neutrophils, eosinophils, T-cells, monocytes and fibroblasts in the bronchial mucosa it should be possible to investigate the cellular basis of asthma, both in relation to clinical patterns of disease and their response to treatment. With such information it may well be possible that a definition of asthma could be arrived at based on pathology rather than function alone.