In previous articles, I have described the technic for x-ray examination of the sinuses which I believe desirable and necessary for dependable diagnosis. I will not repeat that discussion here further than to say that films of the highest grade of technical excellence are necessary, and that views must be taken in all positions that might give any additional information. Under the most favorable circumstances, there are enough inherent potential sources of error in sinus diagnosis without adding the unnecessary ones of poor technic. Having obtained satisfactory films, it is still necessary to have some knowledge of the clinical aspects of the case if gross errors of interpretation are to be avoided. For instance, an antrum filled with polypoid tissue may look exactly the same on the film as it would if filled with pus. The knowledge that polypi were present in the nose, however, or that pus was draining from the antrum, would put an entirely different aspect on the situation. If the radiologist who interprets the films does not have this information, then the rhinologist who performs the physical examination must correlate his findings with the x-ray report, with a full understanding of the fundamental limitations of the x-ray and without criticism of his colleague. In this field of diagnosis co-operation is probably more important than in any other. For purposes of discussion, sinusitis may be classified as follows: Acute 1. Catarrhal Hay fever Cold in the head 2. Suppurative Cold in the head Contagious diseases Acute fulminating Chronic 1. Suppurative Recurring attacks More or less constant draining 2. Hyperplastic Allergic Edema of membrane Polypi Cysts Complicating suppuration Acute Sinusitis Acute catarrhal sinusitis is probably not often recognized, because there is little occasion to examine the sinuses roentgenologically in such cases. I believe any extended investigation of hay fever patients by x-ray would reveal a high percentage of opaque sinuses, particularly the maxillary, due to swelling and edema of the mucous membrane continuous with that in the nose. The same thing applies to the acute stage of a cold in the head. Interest in this type of sinusitis is, how-ever, largely academic, as it has little clinical significance. Acute suppurative sinusitis usually results either from the direct spread of infection from the nasal cavity in a head cold or as a complication of an infectious disease. Of these causes, influenza is undoubtedly the most common, but the acute contagious diseases of childhood are also frequent offenders.