OBJECTIVE The relationship between temporomandibular joint (TMJ) effusion and TMJ disorders is controversial. The frequency of TMJ effusion has varied as shown in previous studies. Furthermore, though some authors have suggested a correlation between TMJ effusion and pain, others question the relationship. In order to clarify the relationship, it is necessary to quantify the degree of effusion and thoroughly investigate its relationship to other factors. The purpose of this study was to analyze the amount of TMJ fluid present in joints with TMJ disorders and to see how TMJ effusion is related to TMJ status and pain. METHODS We studied 577 joints in 293 patients referred to us for magnetic resonance imaging. The joints were divided into painful and nonpainful categories and also classified according to Westesson's criteria for the status of the TMJ. The grading system of Larheim et al was used for categorizing the amount of fluid. Statistical methods were used for analyzing the relationship between TMJ fluid and TMJ status and pain. RESULTS TMJs with disk displacement without reduction showed the largest amount of fluid while TMJs with normal superior disk position showed the least fluid (Kruskal-Wallis test, P<0.001; Scheffe test, P<0.001). There was a significant difference in the amount of the fluid between painful and nonpainful joints in the group of disk displacement without reduction (Wilcoxon rank sum test, P<0.001). No significant differences were found between other groups. CONCLUSIONS Joint effusion is likely to appear in painful TMJs with disk displacement without reduction. Joint effusion may be an abnormal entity just suited to joints with disk displacement without reduction.