While thrombosis of the lateral sinus is the intracranial complication most frequently encountered after operations for mastoiditis, the number of cases observed by the average otologist is so small that he feels obliged to draw on the larger collective experience of his confr\l=e'\resin formulating criteria for the management of this serious condition.When he consults the literature, he finds many excellent reports of cases and the expression of a wide diversity of opinion regarding the efficacy of the surgical procedures used to combat thrombosis of the lateral sinus. It is difficult, if not impossible, to draw definite conclusions, for there are records which seem to substantiate each contrasting opinion.Since the individual series of cases reported are, as a rule, small, it occurred to me that to obtain the recent records from various parts of the country on the incidence of thrombosis of the lateral sinus following operations for mastoiditis and the outcome might yield valuable information concerning this complication. So far as I have been able to determine, such records have not been assembled. A letter to the medical department of the Metropolitan Life Insurance Company brought the reply that that organization has no figures on the incidence of thrombosis of the lateral sinus following mastoid operations in various parts of the country and knew of no one who could furnish such information.Approximately a thousand questionnaires were sent to leading otologists and hospitals in the United States and Canada. To this inquiry three hundred and forty-three replies were received. These came from physicians and hospitals in thirty-nine states, Hawaii and the District of Columbia, as well as five provinces in Canada (chart).Of this number, one hundred and seventy-eight gave actual figures concerning the number of operations on the mastoid and the number of cases in which thrombosis of the lateral sinus occurred. Nineteen more gave information concerning the number of cases of thrombosis of the lateral sinus or the percentage incidence of the complication without Presented as a candidate's