Pneumosinus dilatans is a rare condition with the dilatation of aerated paranasal sinuses. We present two cases and discuss its aetiology, the radiological classification of enlarged aerated sinuses, and its treatment. One much quoted hypothesis for the cause of this condition has been that it is due to a ‘ball valve’ effect of mucosal closure at the frontal recess. However, one of our subjects has started, and done, a considerable amount of subaqua diving since presenting with their condition and has had no symptoms on diving, or progression of their frontal swelling. This observation, along with the fact that retained secretions are not seen within the sinuses in this condition, raises doubt about the theory that a one-way valve is responsible.