Acute and chronic infections cause morphological changes in the respiratory mucosa. The ultrastructure of human respiratory mucosa was studied by scanning electron microscopy from the maxillary sinuses of 28 patients, with chronic sinusitis, from middle turbinates of 60 patients, with recurrent respiratory infections, and from healthy sphenoidal sinuses of 31 patients. A loss of ciliated cells and an increasing number of nonciliated columnar cells with microvilli were seen in 62 per cent of the maxillary sinus mucosa. Ciliary disorientation was seen in 81 per cent of the chronically infected sinus mucosa and eight per cent in the healthy sphenoidal sinuses. Also metaplasia and extrusion of epithelial cells were prominent in chronic infections. Compound cilia were seen in 52 per cent of the samples from patients with chronic sinusitis and in 31 per cent of the healthy sphenoidal sinuses. Short cilia were often seen in infected mucosa indicating ciliogenesis.