Recent investigations have identified the viscous middle ear effusion in secretory otitis media (S.O.M.) as being of the same nature as true mucus--the "backbone" of which is glycoprotein. The source of the mucus is mucus-producing cells in greater number than found in the normal middle ear lining. The pathological state which leads to excessive mucus production and maturation of numerous mucus cells is an inflammation associated with deficient ventilation of the middle ear. In the presence of inflammation, especially when CO2 tension is high, mucosal stem cells differentiate metaplastically into numerous mucus-producing cells, as demonstrated by us in tissue cultures. The higher CO2 tension in S.O.M. is due to eustacian tube insufficiency (not blockage) which is a smaller than normal amount of air introduced into the middle ear per unit of time. Ventilating tubes restore CO2 tension to normal and equalize the slight barometric negative pressure (average 3.35 H2O)--the latter being related to mucociliary clearance. Evidence for an allergic factor as a pathogen is also not available. The mucus in the middle ear is probably not noxious by itself and ears should be ventilated surgically only when hearing is below social level perception. Most S.O.M. ears will clear sooner or later spontaneously. A minority of these ears will progress with time into atelectatic ears and some might perforate later on.