The prognostic significance of positive peritoneal cytology in endometrial carcinoma has led to the incorporation of peritoneal cytology into the current FIGO staging system. While cytology was shown to be prognostically relevant in patients with stage II and III disease, conflicting data exists about its significance in patients who would have been stage I but were classified as stage III solely and exclusively on the basis of positive peritoneal cytology (clinical stage I). Analysis was based on the data of 369 consecutive patients with clinical stage I endometrioid adenocarcinoma of the endometrium. Standard treatment consisted of an abdominal total hysterectomy, bilateral salpingo-oophorectomy with or without pelvic lymph node dissection. Peritoneal cytology was obtained at laparotomy by peritoneal washing of the pouch of Douglas and was considered positive if malignant cells could be detected regardless of the number of malignant cells present. Disease-free survival (DFS) was considered the primary statistical endpoint. In 13/369 (3.5%) patients, positive peritoneal cytology was found. The median follow-up was 29 months and 15 recurrences occurred. Peritoneal cytology was independent of the depth of myometrial invasion and the grade of tumour differentiation. Patients with negative washings had a DFS of 96% at 36 months compared with 67% for patients with positive washings (log-rank P<0.001). The presence of positive peritoneal cytology in patients with clinically stage I endometrioid adenocarcinoma of the endometrium is considered an adverse prognostic factor.