Conventional clinical staging of cervical cancer is subjective because it is based on palpatory findings and inadequate because it cannot assess the single most important prognostic factor--tumor size. To determine the exactitude of in vivo MRI measurements of tumor volume, 22 patients with invasive cervical cancer were studied before surgery. The volumes obtained by MRI correlated well (r = 0.983) with those obtained by histomorphometric analysis of the surgical specimens, but only weakly with clinical stage. MRI may provide a basis for precise classification of cervical cancer and for objective comparison of surgery and radiotherapy.