In this paper two cases are reported in which spontaneous entry of air into the head appears to have occurred through a hypercellular mastoid air cell system. In both these cases forceful sneezing and nose blowing were considered contributory factors. They underwent surgical repair of the bony defects which, combined with less vigorous nose blowing, has affected a successful repair. The aetiology of pneumocephalus is discussed and a review of the pertinent literature is also presented.