This report highlights the importance of accurate interpretation of even small intraoperative capnography changes. In this case, an otherwise unexplained sudden small decrease in end-expiratory carbon dioxide tension (P_ETCO₂) led to the detection of a deep venous thrombus. During the surgery of a 34-year-old woman with a known carcinoma of the corpus uteri, who was scheduled for paraaortal lymphadenectomy, a sudden decrease in carbon dioxide tension occurred. In careful exploration of the surgical field to rule out a thromboembolic event, the surgeons noticed an induration of the right iliac vein. A consulting vascular surgeon exposed a right-sided, irregular, double-lumen, common iliac vein with a thrombus the tip of which floated in the inferior vena cava. Following complete thrombectomy, the procedure was accomplished without further adverse events.