Obiective To explore the causes of infants with premature thelarche(PT). Methods A total of 351 infants (3 months~3 years old) with PT, an average age of 1.4 years old, from the endorctinology department of Zhengzhou Children's Hospital, were included into the oberserved groups.130 cases, an average age of 1.3 years old (3 months to 3 years old), from the in-patient department of Zhengzhou Children's Hospital, were included into the control groups.All cases were accepted the questionnaire survey and sex hormones test.Cases of the oberserved groups were followed up more than 2 years. Results 1)Breast development regressed in 296 cases of the oberserved groups within 1 year.The rate of regression was 96.7%.2)The level of estradiol in the oberserved groups was higher than the control groups, especially in the groups of 6 months ~1 year old and 1~3 years old(P<0.05).3)There were no significant differences between oberserved groups and control groups in birth weight, taking progesterone and pesticides exposure during mother pregnancy.While there were significant differences between oberserved groups and control groups in artificial feeding, using whitening skin care products of guardians and living at suburban district of Zhengzhou (P<0.05). Conclusions Maybe the major cause of infants with PT is highly sensitive of exogenous estrogen hormones or environmental pollutants base on the genetic susceptibility.A few infants with breast development have organic diseases should be follow-up for a long terms.