Mucoid degeneration of the anterior cruciate ligament (ACL) is a rare cause of anterior knee pain (AKP). Some case reports have been published; however, it is difficult to diagnose and is often underdiagnosed or misdiagnosed because of its pathophysiological ambiguity. We report a rare case of a patient diagnosed with bilateral mucoid degeneration of the ACL with AKP and no limited joint range of motion (ROM). A 59-year-old man with spontaneous right AKP was admitted to our hospital. He first underwent arthroscopic resection of the thickened medial plica protruding far into the medial patellofemoral joint (PFJ) but felt little effectiveness thereafter. He then had an arthroscopic release of the lateral patellar retinaculum because of valgus knee and patellar instability, which resulted in only temporary improvement. Then, the AKP relapsed, this time with limitations in the ROM. Magnetic resonance imaging (MRI0 showed a diffuse, thickened ACL with a high inhomogeneous intensity in the T2-weighted and proton density weighted images and which looked similar to a celery stalk. Based on the patient's history and MRI findings, we suspected mucoid degeneration of the ACL and subsequently performed arthroscopic excision. At the same time, AKP appeared on the other side. Since the MRI demonstrated a similar celery stalk image as before, the same operation was performed on this side, as well. Finally, AKP and the limitation of the ROM were relieved approximately one month after surgery. Due to the patient only suffering from AKP with a preserved ROM, it took about 14 months to diagnose this disease. It should, therefore, always be considered in cases of AKP alone.