Based on a review of the literature and analysis of six new cases, three categories of enlarged, aerated sinuses are defined, namely: hypersinus, pneumosinus dilatans, and pneumocele. The information gained by our study of the area variation of the frontal sinuses in a normal population (part I of this paper) was utilized to define the term hypersinus. In this condition there is generalized enlargement of the sinus beyond the upper limit of normal in an asymptomatic patient. The principal difference between pneumosinus dilatans and a pneumocele is the presence of bony thinning or erosion in the latter entity. The clinical findings and the possible etiologies of these conditions are discussed.