Patient: Female, 58Final Diagnosis: Breast cancerSymptoms: NoneMedication: NoneClinical Procedure: Insertion of a peripherally inserted central catheter (PICC)Specialty: AnesthesiologyObjective:Unusual or unexpected effect of treatmentBackground:A peripherally inserted central catheter (PICC) causes few serious complications but can be malpositioned. To avoid malpositioning, ultrasound guidance is widely used. Here, we report the case of a patient who received a PICC that was inserted under ultrasound guidance, but the catheter tip accidentally entered the right inferior thyroid vein.Case Report:A 58-year-old woman was scheduled for reconstructive mammoplasty. After general anesthesia, a PICC was inserted via the right basilic vein. The PICC was inserted under guidance using a portable ultrasound machine with a high-frequency linear transducer. The tip of the guide wire and catheter were confirmed by ultrasound to be in the right subclavian vein, not in the right internal jugular vein, during insertion. However, the chest X-ray performed after the PICC insertion showed that the catheter had moved into the right inferior thyroid vein.Conclusions:Malpositioning of a PICC can occur into any small vein. Ultrasound should be used not only to avoid malpositioning into the IJV, but also to confirm the proper position of the catheter tip during PICC insertion.