OBJECTIVE The aim of the present study was to evaluate the association between temporomandibular joint (TMJ) effusion and disk displacement by means of magnetic resonance (MR) imaging. METHODS One hundred and ninety-four patients were included in the study and underwent a bilateral MR of the TMJ at both closed mouth and maximum mouth opening positions. The association between TMJ effusion and disk displacement with or without reduction was assessed by means of 2 x 2 contingency tables and a permutation test for a categorical variable. RESULTS The results showed a statistically significant association between joint effusion and disk displacement without reduction (DDNR) (P = .008). There was no statistically significant association between TMJ effusion and normal disk position (P = .99) or disk displacement with reduction (DDR) (P = .43). CONCLUSIONS Although these results show a significant association between joint effusion and disk displacement without reduction, there remains uncertainty as to if the nonreducing displacement causes the effusion or vice versa. The present investigation also pointed out the absence of association between reducing disk displacement and effusion. These findings have to be put into relation with clinical and hystological findings.