Differential enzymic analyses of the erythrocyte glutamic-oxaloacetic transaminase and the erythrocyte glutathione reductase of a patient with a 3-yr history of the carpal tunnel syndrome (CTS) revealed high deficiencies of both vitamin B-6 and riboflavin as based on approximately equal to 30% levels of the specific activities of these enzymes. Riboflavin for 5 months caused nearly complete disappearance of the CTS and caused no change in the specific activity of erythrocyte glutamic-oxaloacetic transaminase. Combined riboflavin and pyridoxine treatment increased (P less than 0.001) the specific activities of erythrocyte glutathione reductase and erythrocyte glutamic-oxaloacetic transaminase to normal levels with total disappearance of the CTS. Objectively, the strength of pinch of both hands increased (P less than 0.001) on treatment with riboflavin and further increased (P less than 0.001) on the combined treatment. For the first time, a significant riboflavin deficiency has been found to be related to CTS. Riboflavin therapy was effective biochemically, subjectively, and objectively, and riboflavin and pyridoxine were even more effective when concomitantly administered.