The plica synovialis mediopatellaris, a so-called shelf, is found in 20%-50% of normal knees. A shelf syndrome with pain or functional injury is uncommon, however, and cases that occur during the adolescent growth phase are rare. We report a rare case of a bilateral shelf syndrome that developed simultaneously in both knees. The patient was a 13-year-old girl who presented with bilateral knee pain and severely decreased range of motion. The symptoms were not alleviated after internal therapy, so arthroscopy was performed. It showed a white hypertrophied synovialis mediopatellaris, a shelf from the suprapatellar pouch, in both knees. The shelf had an opening between the free margin and the insertion point, giving rise to a synovialis mediopatellaris with a detached structure (type D according to the Sakakibara classification). The cause of the pain was thought to be impingement of the synovialis mediopatellaris. The shelf was ablated by arthroscopy using a holmium-YAG laser. Since the patient was a 13-year-old girl in the growth phase, it is possible that the growth resulted in tension on the shelf remnant bilaterally, intruding into the medial aspect of the patellofemoral joint and increasing the tension, resulting in the symptoms reported.