We have previously reported that simple hysterectomy is adequate therapy for microinvasive carcinoma of the cervix with less than 3 mm stromal invasion (Yajima, A., et al, Am. J. Obstet. Gynecol. 135, 685-688, 1979). However, whether it is possible to effectively treat patients with deeper stromal invasion remained in question. A policy for surgical treatment in patients with 3 to 5 mm invasion has not been standardized. Although radical hysterectomy is usually performed, some authors insist that patients with 3 to 5 mm invasion can be safely treated conservatively. Patients treated with conservative surgery should be carefully assessed because cases with confluent invasion or lymph-vascular space invasion have a greater potential for lymph node metastasis and carry a worse prognosis. Conservative treatment in this study was limited to patients with microinvasive carcinoma with 3 to 5 mm invasion, stage IB occult patients (classified by the FIGO 1973 staging system). The patients were preoperatively diagnosed with either carcinoma in situ or microinvasive carcinoma, and treated by conservative surgery (conization, simple hysterectomy, or extended hysterectomy). Twenty-two patients with 3 to 5 mm invasion but without additional risk factors, i.e., confluent invasion or lymph-vascular space involvement, were followed without postoperative radiation therapy. All patients survived except one (who died of an intercurrent disease) and there were no signs of recurrence of serious surgical complications. The present study with stage IB "occ" subjects suggests that conservative therapy rather than radical hysterectomy can be used in a very carefully selected subset of patients, i.e., in cases with 3 to 5 mm microinvasive carcinoma without additional risk factors, such as confluent invasion and lymph-vascular space invasion.