To examine the influence of inhaled steroids on the bronchial mucosa, biopsies from six patients with severe bronchial asthma were studied before and after ten years of daily treatment with inhaled steroids. Biopsies from six healthy subjects were also examined. In the biopsies taken from the asthmatic patients before treatment there was a significant increase in inflammatory cell numbers compared with the biopsies from the control subjects. In all patients scanning electron microscopy showed a reduced coverage by cilia. Squamous cell metaplasia was seen in two patients. After ten years of treatment the number of inflammatory cells was significantly reduced compared to that before treatment and was not different from the control biopsies. Most of the epithelial cells showed a ciliated surface. Small focal areas with non-ciliated cells could still be seen in four patients. In the two patients with squamous cell metaplasia before treatment, small areas of metaplasia could still be seen. Despite the absence of inflammation and reduced epithelial damage during treatment all patients still had bronchial hyperresponsiveness.