Mass poisoning with an organo-phosphate insecticide occurred at a junior high school in Japan. We collected clinical data from four hospitals, which treated the patients, and investigated this poisoning.
  In late July, the janitor of the school dropped an undiluted solution of 40% methidathion insecticide into the nest of ant near the classroom between 10:00 and 11:00 a.m The volatilized gas spread into the classroom, and many students presented some symptoms of poisonig, 16 students were taken to neighborhood hospitals around noon.
  Of the 16 patients, 13 complained of headache, 11 of nausea, and 4 of dizziness. One patient vomited twice. The serum cholinesterase activities, the pupil diameters and the light reflexes of the patients were normal. Three patients were hospitalized one night for observation. All patients did not get sever poisoning, and recovered and had no sequela.
   It has to be admonished against spraying an undiluted solution of high toxic insecticide near the classroom, and it is necessary to take stern measures to prevent another such accident.