The clinical investigation regarding to vitamin B(1) metabolism was performed on rheumatoid arthritis during one month of admission, during which period the various antirheumatic agents were given to these patients. 1) The total thiamine levels of blood in rheumatoid arthritis was low compared with normal healthy people, specifically low in ester type suggesting the decrease of phosphorylation. There was also present the advancements of erythrocyte sedimentation rate and disturbed function of liver with advance of "stage" and "class" of this disease. The decrease of rate of phosphorylation was seemed to be parallel to them. 2) It is able to consider that the vitamin B(1) deficient of rheumatoid arthritis is due to the decrease of vitamin B(1) level in blood resulted from the lower efficiency of vitamin utilization. 3) It is concluded from the observations of clinical symptoms at the rheumatoid arthritis that as the temporary elevation of vitamin B(1) level in blood by administration of large dosage of vitamin B(1) appears to be insufficient in analgetic activity for rheumatoid arthritis, so it seems better in the treatment by corticoids with small dosage of vitamin B(1) administration for long period rather than by large dosage for short period. 4) For the improvement of vitamin B(1) metabolism, thiamine propyl disulfide and cocarboxylase are more effective than the other, and the combind administration of vitamin B(1) with ATP was effective, specifically for the elevation of ester type of vitamin B(1). It will be concluded that the protecting treatment of liver function is also useful not only for elevation of the value of vitamin B(1) in blood but also for the improvement of general symptoms of rheumatoid arthritis.