Abstract: Objective: To study the prevailing characteristics of the infantile Mycoplasma pneumoniae infection,its relations with the age,season,sex and the initial symptoms after MP infection.Methods: The MP-Ab in the serum of some children under the age of 14 who were treated in our hospital from Jan.1999 to Dec.2004 was measure by the method of passive particle agglutination.Results: The positive rate of!MP-Ab had certain laws:there was significant difference between the four seasons(P<0.01);the difference was significant between the male and the female(P<0.005);It was commonly seen in the preschool children(32.24%) and the school children(40.63%).The symptoms outside the lung caused by MP infection tended to increase.Conclusions: The MP infection is closely related with the age,season and sex.The lesion outside the lung caused by MP infection has the tendency to increase in recent years.