Diseases causing trismus or pain of the temporomandibular joint (TMJ) include temporomandibular joint disorders (TMJD), trauma, tumors and an inflammation. This report describes two cases of masticator space abscesses causing trismus and TMJ pain, which were initially diagnosed as TMJD. The first case was a 62-year-old female suffering from trismus and right-TMJ pain without swelling or redness, which was diagnosed as TMJD with MRI (TMJ region) findings of bilateral anterior disc displacement without reduction. However, five days later the right-TMJ pain became more severe and MRI (maxillofacial region) revealed a right masticator space abscess. She was treated with antibiotics and underwent an intraoral incision following hospitalization. She recovered well and was discharged from the hospital on the 15th day. The second case was a 68-year-old female having trismus and left-TMJ pain, which was diagnosed as TMJD at another hospital. She received instructions for jaw opening training, but visited our hospital because her symptoms were not improved. At the first visit, the mouth opening range was 18 mm and an anterior dislocation of the left condylar process was found by X-ray imaging. CT-scan revealed an abscess in the left masticator space. Therefore an intraoral incision was performed under general anesthesia and antibiotics were administered. She recovered well and was discharged from hospital on the 12th day. When we treat TMJD, we must keep in mind the possibility of the inapparent presence of some unseen inflammation, especially an abscess formation in the masticator space.