One thousand and one disorders of the ear, nose and sinuses in 650 SCUBA divers were analysed for anatomical distribution and causation. The outer, middle and inner ears made up 64.6% with a surprising 23.9% being related to the lower jaw, its teeth, attached muscles and the temporo-mandibular joint. Of the remaining, 3.1% were related to the nose, 6.6% to the sinuses and there was a miscellaneous group of 1.8%. Most outer ear infections seemed to be self-inflicted whilst the middle ear conditions were due to dysfunction of the Eustachian tube. The middle ear conditions were prevented by reducing nasal congestion as by cauterising the inferior turbinates of the nose, or by giving counselling in the technique of clearing the ears. Apart from Eustachian tube dysfunction in some, there is no clear lead as to causation of the inner ear complaints. Nasal treatment also relieves blockage of the ostia of the sinuses and thus sinus barotrauma. Overall the commonest preventable cause appears to be nasal congestion due to a variable mixture of infective or vasomotor rhinitis, allergic rhinitis, unilateral hypertrophy and a deviated nasal septum.