In 1995, the International Federation of Gynecology and Obstetrics (FIGO) defined stage IA cervical cancer as superficially invasive tumors with a horizontal spread of 7 mm or less; stage IA1 tumors have stromal invasion of 3 mm or less, and stage IA2 tumors have 35 mm of stromal invasion. No consideration is given to vascular space involvement or to special invasion patterns. This study was designed to evaluate the adequacy of these classifications. Four hundred two patients underwent surgical treatment for FIGO stage I squamous cell cancer of the cervix at the Cancer Institute Hospital in Tokyo, Japan, between 1970 and 1995. Those whose surgical specimens showed stromal invasion of 5 mm or less served as the study subjects. One hundred fifty-five women were treated with radical hysterectomy, and 247 were treated with simple hysterectomy. Patients who underwent simple hysterectomy and were subsequently found to have invasive disease greater than 3 mm received external radiation therapy. Three hundred thirty women had stage IA disease according to the FIGO definition; 297 of these were stage IA1, and 33 were stage IA2. Of the 72 women with horizontal tumor spread of more than 7 mm (stage IB), 20 had tumor invasion of 3.0 mm or less, and 52 had tumor invasion of 3.1 to 5.0 mm. Seventy-two of the 297 stage IA1 patients underwent radical hysterectomy, and 225 were treated with simple hysterectomy. Six positive lymph nodes were found among the 155 patients who had undergone radical hysterectomy (stage IA1, 1 of 72; stage IA2, 1 of 29; stage IB, 4 of 54). Lymphatic-vascular space involvement was seen in three of these six patients, all of whom had horizontal tumor spread greater than 7 mm. Of the 297 stage IA1 patients, 5 (1.7 percent) had lymphatic-vascular space involvement, and confluent invasion was seen in 13 (4.4 percent). Of the 33 stage IA2 patients, 7 (21.2 percent) had lymphatic-vascular space involvement, and 12 ( 36.3 percent) had confluent invasion. In the entire group of 402 patients, there were four recurrences. One was in a stage IA2 patient who had a tumor that was 3.5 mm deep and 4.5 mm wide. Three occurred in patients with stage IB disease, whose tumors measured 1.5 × 7.3 mm, 3.5 × 8.0 mm, and 4.0 × 9.0 mm, respectively. The only patient who survived is the woman with 1.5-mm-deep tumor invasion. She was treated with radiation therapy for her recurrence and currently has no evidence of disease. Gynecol Oncol 1999;74:165–169