From the biological significance of thiamine, it is conceivable that the fate of thiamine in the living body is related to the occurrence of chronic sinusitis to some extent.With this belief, the thiamine contents in the blood and mucous membrane of patients with chronic sinusitis were estimated by the thirochrome fluorescent method and the distribution of thiamine in the mucous membrane was observed histologically.The values obtained were then compared with the pathological changes in the mucous membr-ane. On one hand, employing rabbits modified experiments on allergic and bacterial sinusitis which are believed to play an important role in the etiology of chronic sinusitis were conducted, and from the dirtribution of thiamine in the blood and mucosa or liver, the difference in thiamine metablism was compared.Again the changes in thiamine distribution in blood and mucosa during liver obstruction were examined.The results obtained were as follows.The thiamine content in blood of patients with chronic sinusitis showed a tendency to decrease slightly in both combined from and free from when compared with the controls.The differences in grade of decrease according to the pathological changes of chronic sinusitis were almost negligiable, but in cases with round cell infiltration in the moderate degree or more in the mucous membrane there were a decrease of total thiamine and an increase of the free form.However, total thiamine content increased markedly in the purulent form when compared with the catarrahl initial inflammatory stage, while it was especially low in the fibrous type which is believed to represent the quiescent stage.The difference in free thiamine according to pathological changes in the mucous membrane was slight.The thiamine distributed in the mucous membrane of human chronic sinusitis consisted mostly of the combined form, while the intensity of chiochrome fluorescence was strong in the epithelial layer, glandular cells, walls of vessels and parts with round cell infiltration, while it was very weak in parts with haemorrhage, edema and interstitial connective tissues.Concerning with relation to pathological changes, an increase of total thiamine and a relative decrease of free thiamine were recognized in cases which indicated enhancement of metablism.In experiments with rabbits, the thiamine content in blood and the intensity of the thiochrome fluorescence in the mucous membrane tended to fall, whether the stress is allergic or bacterial.These findings differ somewhat according to the strength of stress, and the disturbance of thia-mine metabolism in those elicited or those taking a long course was greater than in groups sensi-tized or groups taking a short course following bacterial invasion.In these cases there were a slight disturbance in the liver, a fall in hepatic function and a decrease of thiamine content in liver cells.On one hand, in experimental liver obstruction, the esterification in liver cells and thiamiae in blood and mucosa also showed evident falls.Thiamine is not only indispensable for metabolism in the living body, but its deficiency induces-a tendency for inflammatory or allergic changes to occur with ease.From the experimental results mentioned above, it is believed that thiamine deficiency in the mucosa and the disturbances in liver function produce a vicious cycle and from the local disturb-ance in metabolism, there results delay of healing; and these may be said to influence greatly the esta blishment of chronic sinus diseases.