OBJECTIVE: Previous studies on prognostic factors in ovarian tumors of low malignant potential (LMP) were too small for robust conclusions. We examined the prognostic impact of preoperative serum CA125 >/= 50 U/ml levels in patients diagnosed with ovarian LMP tumors in a large multinational cohort. METHODS: This retrospective study included 940 patients with ovarian LMP tumors diagnosed between 1985 and 2008 at six gynecologic cancer centers. Patients either had radical treatment (bilateral salpingo-oophorectomy with or without hysterectomy) or conservative, fertility-sparing treatment. Multivariate Cox proportional hazard models were used to determine independent prognostic factors for disease-free (DFS) and overall survival (OS). Based on receiver operating characteristic curve (ROC), a preoperative serum CA125 level >/= 50 U/ml was considered "elevated". RESULTS: CA125 was more often elevated in serous than in mucinous tumors and in advanced FIGO stages (2 to 4) compared to stage 1. DFS at 5 years was 89% and 95% in patients with elevated and normal CA125 levels (p