A prospective study was conducted on 50 consecutive patients with stage I endometrial cancer who had primary surgical treatment. The purpose of the study was to assess the value of magnetic resonance imaging (MRI) for accurate staging of early disease and determination of myometrial invasion. Features identified by MRI were correlated with surgical pathology. Preliminary MRI results provided additional valuable information. All but one of 18 patients with histologically proven deep myometrial invasion were predicted preoperatively by MRI. Of 17 patients with detached fragments of malignant tissue in the endocervical curettage (ECC) but with results inconclusive for actual cervical invasion, MRI revealed all three patients with true cervical tissue involvement. Magnetic resonance imaging detected all six patients with gross extrauterine spread and also precisely measured uterine enlargement by myomata. The extent and location of tumor growth in the uterus could be mapped out in the majority of cases. Based on these findings, a pretreatment MRI scan of the pelvis in presumably stage I endometrial carcinoma resulted in an advance in staging in 18% of the patients, and accurately predicted deep myometrial invasion in 94% of the cases. Inclusion of MRI in the routine work-up in stage I endometrial carcinoma should be considered for proper clinical staging, particularly in patients with a positive but nondiagnostic ECC, uterine papillary serous carcinoma, or grade 3 tumor.