SUMMARYA 15‐year‐old boy with compulsive hand washing and fear of contamination was treated by response prevention.Each series of the response prevention was experimentally divided into four periods: total response prevention, partial response prevention, free hand washing under noticed observation, and free from prevention and free from observation periods.After nine repeats of the series he became free of the symptoms.Temporal increase of fear and appearance of “freezing” phenomenon were observed in the total response prevention periods.Besides decrease in fear and hand washing, there was observed a change of his thought on symptoms with progress of the treatment.Based on these observations and results, the mechanism of treatment of response prevention was di