We have developed an accurate method to measure vocal tract (VT) shape and length from magnetic resonance (MR) images acquired during sustained phonation of Japanese vowels. The 3-dimensional (3D) VT shape was reconstructed by using coronal MR images for the oral cavity and axial MR images for the pharyngeal and glottal regions. A mid-sagittal image was used as a reference for the reconstruction. MR images of dental impressions of a subject were also incorporated into the reconstruction. All the MR images were directly transferred to a workstation where an intercative program was used to measure the VT shapes. Three Japanese subjects, a child, a female and a male, participated in the experiment. Formant frequencies were computed from the 3D VT shapes which were measured for the three subjects assuming one-dimensional sound wave propagation through the vocal tract. The first three formant frequencies were compared with the ones measured directly from real voice. Except for a few cases, differences between the two measurements were all less than the difference limen (DL) of the formant frequencies.