OBJECTIVE To assess the normal position of the temporomandibular joint (TMJ) disk relative to the condyle by using coronal magnetic resonance (MR) imaging in asymptomatic volunteers. METHODS This study was approved by the review committee for human research, and all subjects signed an informed consent form. Thirty symptom-free volunteers without histories of TMJ disorders underwent standardized clinical examinations. Afterward, bilateral sagittal oblique and coronal oblique MR images were acquired with the patient's mouth opened and closed. The coronal oblique opened- and closed-mouth images were analyzed by using computer software. The medial and lateral edges of both the TMJ disk and the condyle were marked for these imaging examinations by using the section through the posterior 3 mm of the disk. To eliminate the effect of different magnifications and/or distortions, the distance between these points was measured automatically and divided by the largest mediolateral dimensions of the condyle. To assess the reliability of the measurements, four observers evaluated the position of the disk in the coronal plane. To assess the changes in position of the posterior 3 mm of the disk in the coronal plane in the closed- and opened-mouth positions, the Wilcoxon signed rank test for matched pairs was used. Interobserver measurement reliability was evaluated by using interclass correlation coefficients (ICCs). RESULTS Analysis of the coronal closed-mouth disk position revealed a medial position of the TMJ disk relative to the condyle in 11 (21%) of 52 analyzed joints. In the opened-mouth position, the medial location of the disk was more frequent: 29 (85%) of 34 analyzed joints exhibited a medial position of the disk relative to the condyle in this plane. This increasingly medial position of the disk was statistically significant (P < or = .001). Measurement reliability assessment revealed sufficient results (ICC > or = 0.7). CONCLUSIONS At both closed- and opened-mouth MR imaging, a medially located TMJ disk seems to be within the normal range of variation. The disk seems to shift even more medially when the mouth is opened.