Within this research, computer simulations of pattern alterations created with CAD/CAM Lectra Modaris sub-module, 3D Fit, were explored. Research tasks performed within it give opportunity to make conclusions regarding causes of pattern alterations, their depiction in computer simulation, measuring possibilities and thereby the prevention of the causes of such alterations thus easing the work of pattern maker. General information regarding the integration of three-dimensional setting into clothing design has been gathered. Summary regarding 3D clothing design/ virtual try-on software; the constructive defects of clothing have been described - both their causes and means of prevention is given. Processes of development of model construction and virtual fitting of upper-body based clothing construction have been described. Computer simulations of constructive defects of clothing have been performed, the results obtained have been gathered and the observations have been described.