Percutaneous endoscopic discectomy (PED) for lumbar disc herniation is gaining popularity with the transforaminal (TF) approach preferred because it allows surgery under local anesthesia and preserves the spinal muscles. Although this procedure has some characteristic complications, it is rare for PED to be converted to conventional open surgery due to worsening of symptoms intraoperatively. Here, we report PED via the TF approach that required conversion to open surgery. A 20-year-old man with a large disc herniation at L3/4 developed severe progressive leg pain and muscle weakness of the left leg intraoperatively. Magnetic resonance imaging revealed that the size of the herniation was unchanged and the endoscope did not reach the herniated mass. We converted to open surgery, and the patient's postoperative course was favorable. We discuss the reasons for failure of the approach and suggest planning for an appropriate foraminoplasty to avoid the potential need for conversion to open surgery.