A twin study is an excellent means of assessing the contribution of heritability to motor behaviour. We present a movement video-analysis of a monozygotic twins pair with a motor repertoire which is almost totally constituted by persistent and subcontinuous motor stereotypies. Purpose: The specific aim of this study is to verify the heritable quantum of motor behaviour and to determine which among the motor patterns we analysed are more likely to be conditioned by inheritance. Methods: Stereotyped movements were videotaped in two standardized sessions: at rest and in relation to preordained sensory stimulations. We estimated the concordance index (CI) between the observers to evaluate the reliability of the observations. The validity was accepted as being CI greater than 0.80. Results: The results showed a very high concordance rate (greater than 90%) for all the stereotypies analysed. An almost superimposable trend of the stereotyped movements was found both at rest and in relation to the sensory stimulations. Conclusions: Such strong data suggest that genetic factors have a primary influence on all the movement disorders analysed. This study contributes to a better understanding of the complex relationships between genes and functions. (Contains 1 table and 3 figures.)