The purpose of this study was to investigate the waveform and electrosonographic characteristics of sounds emanating from internal derangement of the temporomandibular joint (TMJ). TMJ sounds were ecorded from 10 joints of normal people (NP), 10 joints from patients with anterior disc isplacement with reduction (DDR) and 20 joints from patients with anterior disc displacement without reduction (DDNR). The sounds were analyzed through fast Fourier transfer methods to observe their waveforms and electrosonographic characteristics. The observations were then used in differentially diagnosing internal derangement. Wave pattern and electrosonography (ESG) differed among the NP, DDR and DDNR groups. There was very little difference in frequency between the sounds from DDR and DDNR, but the amplitude of the DDR sounds was higher than those of DDNR and NP. The sensitivity and specificity of ESG diagnosis for DDR were 77.2% and 93.3%, respectively, while for DDNR they were 81.6% and 64.7%, respectively.