To investigate the clinical characteristics, management and outcome of patients with lung metastasis of invasive mole (IM) before evacuation of hydatidiform mole (HM).Clinical data of patients with hydatidiform mole (HM) and lung metastasis of IM who were diagnosed and treated at Peking Union Medical College Hospital from Jan 2004 to Jan 2006 were analyzed retrospectively. Firstly, clinical characteristics of patients with lung metastasis of IM before evacuation of HM (positive group) were compared with that of patients without lung metastasis before evacuation of HM (negative group); secondly, management and outcome of patients with lung metastasis of IM before evacuation of HM were compared with that of patients as lung metastasis of IM diagnosed in postevacuation follow-up of HM (control group).A total of 37 cases with HM underwent CT scan of the chest before evacuation, 11 cases of which were diagnosed as lung metastasis, accounting for 30%. Compared with negative group, significant increases in positive group were found in gestational age of week [(15.0 +/- 4.0) versus (10.0 +/- 2.5) weeks, P = 0.026], and proportion of complete HM (91% versus 50%, P = 0.027). Between positive group and negative group, no significant differences were found in age, uterine size greater than expected for gestational age, large theca lutein cyst and pre-evacuation serum human chorionic gonadotropin-beta subunit (beta-hCG) level (P > 0.05). Compared with control group, significant decrease in positive group was found in the interval from first evacuation of HM to resolution of serum beta-hCG level, (83 +/- 18) days versus (126 +/- 31) days (P 0.05).Once HM is diagnosed, evacuation should be performed as soon as possible, the later the evacuation begins, the higher the risks of lung metastasis and chemotherapy are. It is not necessary to worry about lung metastasis before evacuation of HM, the outcome of post-chemotherapy is very good.