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Free thiamin and thiamin monophosphate levels were determined by an electrophoretic fluorometric micromethod in plasma and CSF of patients with amyotrophic lateral sclerosis (ALS), alcoholics, and controls. In plasma of patients with ALS as well as in plasma and CSF of alcholics, both thiamin and thiamin monophosphate concentrations were decreased so that the thiamin-thiamin monophosphate (T/TMP) ratio remained unchanged compared with that of controls. In CSF of patients with ALS, however, thiamin monophosphate values decreased much more than thiamin levels, so that the T/TMP ratio was significantly increased. The selective impairment of thiamin monophosphate production by nerve cells is likely to result from the reduction of the activity of thiamin pyrophosphatase, an enzyme synthetized and highly concentratd in the Golgi complex. Thiamin pyrophosphatase is known to diminish in ALS as well as in experimental motor neuronal degeneration or axotomy. Thus, the T/TMP ratio could be taken as an index of the impairment of neuronal protein synthesis in ALS.