BACKGROUND Pressure and waveform at the catheter tip are continuously monitored during catheterization of pulmonary artery to ensure accurate catheter placement. We present a case in which pulmonary venous blood was unexpectedly collected from the pulmonary artery catheter despite pulmonary artery pressure and waveform detection at the catheter tip, and describe the measures taken to correct the catheter placement. CASE PRESENTATION A 74-year-old male underwent mitral valve plasty for cardiac failure caused by mitral valve regurgitation. Preoperative transthoracic echocardiography showed no septal shunt. The pulmonary artery was catheterized through a sheath introducer in the right jugular vein, and the balloon was inflated after insertion of a 15-cm catheter. The catheter was advanced until a pulmonary artery waveform was detected and the pulmonary artery wedge pressure was 21 mmHg at end-expiration. The balloon was deflated and the catheter tip was pulled back 3 cm. Pulmonary artery waveforms and appropriate a and v waves were detected, and transesophageal echocardiography confirmed the location of the catheter tip in the right pulmonary artery. The first collected blood sample had an oxygen partial pressure of 358.8 mmHg, carbon dioxide partial pressure of 20.1 mmHg, and oxygen saturation of 99%, indicating pulmonary venous blood. The pulmonary artery catheter was pulled back 5 cm, but a second blood sample showed the same results. The catheter was pulled back a further 6 cm while the location of the catheter tip was monitored on X-ray fluoroscopy. Blood gas testing through the catheter tip showed oxygen saturation of 84.4 % and oxygen partial pressure of 41.6 mmHg. Surgery was performed uneventfully. Postoperative chest radiographs showed proper placement of the pulmonary artery catheter, but radiographs on postoperative day 1 showed over-insertion, although the insertion length was unchanged. The catheter was removed. The patient was discharged 2 months postoperatively. CONCLUSIONS Our case highlights the fact that the tip of the pulmonary artery catheter can easily advance into a peripheral branch of the pulmonary artery and cause pulmonary venous blood to be sampled instead of pulmonary arterial blood. A variety of monitoring techniques are needed to confirm accurate catheter placement.