The relationship between the respiratory threshold of acetylcholine (Rt-Ach) and the clinical course of asthma was studied in 70 asthmatic patients. RT-Ach was examined by the standard method in Japan. The clinical course of asthma was evaluated according to the daily record of asthmatic symptoms. The observations periods were 1, 6, and 12 months before, and 6 and 12 months after the inhalation test. There was a significant negative correlation between RT-Ach and the total number of days of asthmatic attacks before the inhalation test. However there was no such relationship between the RT-Ach and attacks after the inhalation test. We concluded that RT-Ach might depend on the clinical course of asthma, especially total days of asthmatic attacks before the inhalation test.