Tooth mobility is a sign of the physiological or pathological condition of the periodontium, which consists of the periodontal ligament and alveolar bone, etc. It is possible to establish tooth mobility by observing the tooth movement between pincettes or between the fingers. Further information is gained by placing the fingertips partly on the teeth and partly on the gingivae and also by asking the patient to clench his teeth and move them laterally and protrusively. A pathological mobility should always be related to the radiographic evidence of the pathogenesis and to pocket-depth measurements. Manual examination by trained observers may correspond to the degree of tooth movement, but it requires skill and experience. Measurements of tooth mobility have already been studied, and some static and sinusoidal dynamic measurements have been proposed. The authors have developed an automatic diagnosis system of tooth mobility for clinical application. This system consists of a measuring device for the biomechanical properties of the periodontium and a data analysis unit with a microcomputer. The labial crown of a tooth is randomly vibrated by a portable vibrator with a small impedance head. The mechanical mobility and parameters which give information about visco-elasticity of the periodontium are obtained. The measurement and data analysis take about 1.4s and 15s, respectively. Tooth mobility is determined objectively by discriminant analysis. The mobility triangle figure with mechanical parameters is drawn as a record for visual interpretation. The dentist can operate the diagnosis system by himself. The objective tooth mobility and viscoelastic parameters of the periodontium, which have been difficult to obtain, not only can be determined quickly but can be applied to a longterm observation of tooth mobility. This paper discusses the measuring conditions, the effects of the static preload on the tooth, the vibrating direction and the linearity to input acceleration. The preload on the tooth is 50±5gf, which does not cause pain to the subject and reflects the properties of the periodontal ligament. Vibrating deviation in the gingival-incisal direction has a more adverse effect on tooth mobility than that in the distal-mesial direction. The mechanical mobility is always measured at approximately constant acceleration 0.05G. The coefficient of variation (standard deviation/mean value) in 10 measurements is 6-9%. The physiological variations of tooth mobility over a day and a month, which are difficult to study using the usual measurements, are discussed. The validation of this system is described through clinical application to the evaluations of a healthy and a pathological tooth and some dental implantations.