Activated c-fos binds to jun proteins to form the activation protein 1 (AP-1) transcription factor that regulates cytokine and other proinflammatory genes. c-Fos may play a key role in nasal polyp formation. Glucocorticoids may exert their anti-inflammatory effects through an interaction of glucocorticoid receptors with AP-1 that leads to mutual inactivation of both factors, and a "default" termination of AP-1 mediated gene activation. This may explain the beneficial effects of glucocorticoids in the treatment of nasal polyps.To test this hypothesis in humans in vivo the immunohistochemical expression of c-fos-immunoreactive material (c-fos-irm) was assessed in nasal polyps from eight steroid naive subjects, polyps from eight subjects treated with topical beclomethasone dipropionate (BDP), and normal inferior turbinate nasal mucosa (n = 6).mRNA for c-fos was detected in all nasal polyps and normal mucosa. In contrast, c-fos-irm was present in all steroid naive subjects but in only two of the eight subjects treated with BDP (p = 0.007, two-tailed Fisher's exact test). c-Fos-irm was expressed solely in epithelial cells and glandular structures; it was expressed in normal epithelium and glands, but the staining intensity was low.Glucocorticoids appear to modulate expression of c-fos-irm and possibly AP-1 in human airway epithelial cells in vivo.