Different technical aspects concerning bronchoalveolar lavage (BAL) are still controversial: optimal instillation volume for airway sampling, processing of lavage cells and measurement of solutes are all matter of debate. Data from BAL must then be interpreted cautiously. In asthma and bronchial hyperresponsiveness different studies with BAL seem to agree on a few points such as the presence in the airways of inflammatory cells, predominantly eosinophils and mononuclear cells, and epithelial cells. Macrophages and polymorphs from asthmatic airways are more active in terms of oxygen radical release, this activity being correlated with methacholine responsiveness. In our studies we found a positive correlation between PD20 and lymphocyte numbers. Inflammatory mediators such as arachidonic acid derivatives and PAF have been assessed in BAL fluids in different studies but the precise meaning of those data still remains to be clarified. Corticosteroid treatment can significantly modify the cellular profile in BAL: eosinophils, epithelial cells and mast cells decrease, but lymphocyte numbers increase. Though in its infancy, direct sampling of airways fluids is likely to lead to a much greater understanding of pathophysiological mechanisms in asthma with important therapeutic implications.