It has been reported that joint effusion, the excessive accumulation of joint fluid in and around the joint, is related to temporomandibular joint (TMJ) disorders such as pain and disk displacement. However, there have been no longitudinal studies of this phenomenon. We performed a longitudinal study on the relationship between joint fluid and various pathological disk conditions. The subjects were 17 patients who visited our facility for orthodontic treatment and were diagnosed using MRI as having internal derangement of one or both TMJs (three males and 14 females; age 12-31 years; mean age 20.5 years). MRI was performed before, during, or after treatment for their disorders. We evaluated the relationship between changes in joint fluid in the joint space and the state of the disk, as well as the presence or absence of pain. Joint fluid was evaluated by classifying the extent of high-signal areas in the upper and lower articular cavities on T2-weighted images. The extent of high-signal areas was classified into five levels. Disk displacement and the extent of displacement were evaluated using proton density-weighted images. Statistical analysis was performed using the chi-square test, and differences in the distribution among the groups were examined. Effusion was noted on the first MRI in nine of the eleven joints (81.8%) in which joint fluid decreased on the second MRI (p<0.01). Displacement remained unchanged or worsened in 18 of the 21 joints (85.7%) that showed joint fluid on the first MRI (p<0.01). Pain was alleviated or absent in all joints in which the fluid decreased. These results suggest that joint fluid may be a factor in the outcome of disk recapture treatment as well as in the evaluation of pain.