Thirty-three institutions collaborating in the Gynecologic Oncology Group gathered surgical and pathological data on 1125 patients with primary, previously untreated, histologically confirmed stage I cervical carcinoma with more than 3 mm of invasion who were selected to undergo radical hysterectomy and paraaortic and pelvic lymphadenectomy. Of the 940 eligible, evaluable patients, 732 had squamous carcinoma. Of the study group, 87 (12%) did not undergo radical hysterectomy because of gross disease beyond the uterus or microscopic aortic node involvement documented at exploratory laparotomy. Among the 645 patients undergoing pelvic and paraaortic lymphadenectomy and radical hysterectomy, five risk factors were significantly associated with microscopic pelvic lymph node metastasis: depth of invasion (P = 0.0001), parametrial involvement (P = 0.0001), capillary-lymphatic space invasion (P = 0.0001), tumor grade (P = 0.01), and gross versus occult primary tumor (P = 0.009). The factors identified as independent risk factors for pelvic lymph node metastasis by multivariate analysis were capillary-lymphatic space involvement (P less than 0.0001), depth of invasion (P less than 0.0001), parametrial involvement (P = 0.0005), and age (P = 0.02). The model was used to predict the chance of a patient having nodal metastasis for any combination of risk factors.