About 600 residents and rescue staff were exposed to what was found to be sarin vapor in the city of Matsumoto on June 27, 1994. Fifty-six patients were admitted to hospitals for symptoms of some kind of gas poisoning. We examined the clinical and laboratory findings of 264 patients who were treated at six hospitals. The presence of extremely low cholinesterase values in blood plasma suggested at first that the toxication was caused by an organic phosphorus chemical. But there were many aspects that could not be explained by hitherto-known organic phosphorus toxication. We exchanged information to establish the nature of the substance responsible and to facilitate treatment. The Nagano Prefecture Public Health and Pollution Research Institute reported on July 3 that they found “sarin” and its degraded substances from the air and water at the presumed site of emission of the toxic substance. This paper is based on the report which we submitted to the “Hospital Liaison and Review Committee on the Matsumoto Toxic Gas Poisoning”.Almost all patients admitted to hospitals had severe miosis and tachycardia followed by bradycardia. Recovery to normal red blood cell cholinesterase activity was gradual reaching normal levels during the 3 month period after exposure to the toxic substance. Visual acuity and visual area improved after several weeks. The relationship between subjective symptoms and blood plasma cholinesterase values was examined. Those with such subjective symptoms as headache, fatigue, feverish feeling, visual field abnormalities, poor vision and abnormal subjective sensations had significantly low blood plasma cholinesterase values in the acute phase. Pseudocholinesterase values (ChE%), expressed as a percentage taking the normal minimum value to be 100%, were decreased in 53 of 222 patients (23.9%) examined within 5 days of exposure. A correlation was found between the degree of decline of ChE% and pupil diameter (P<0.01). Creatine kinese (CK%), expressed as a percentage taking the normal maximum value to be 100%, was correlated with ChE% examined within 5 days of exposure.