Method for mounting side doors on motor-vehicle bodies, wherein the doors are connected to the body by means of a pair of hinges each providing a pivot for support and articulation of the door, each hinge consisting of an element for connection to the body and an element for connection to the door, as well as a pin for their mutual rotational articulation, wherein said method comprises the steps of: - detecting a plurality of dimensions measured along coordinates X, Y, Z on the vehicle body and on the doors, with the aid of proper references, for qualifying the geometric conditions of the coupling between the doors and the respective door receiving seats, - determining, on the basis of the detected dimensions, the mounting positions of each pair of hinge elements (CF,CM) to be connected separately to the vehicle body sides and to the door structures, so as to obtain that the pivots for support and articulation of the doors are well aligned with each other and that the geometrical setting of the door is consistent with the respective door receiving seats on the vehicle body sides, according to the assembly project drawing, - mounting the aforementioned hinge elements (CF,CM) on the body in the positions thus determined, said method being characterized in that said step of detecting a plurality of dimensions is made by means of optoelectronic systems, and in that said method comprises the further steps of: - providing the doors with the respective door hinge elements, while making - via software - the system for mounting the door hinge elements to interact with the system for mounting the body hinge elements, in order to substantially ensure consistency with the assembly project drawing, - mounting the doors on the vehicle body by connecting the body side hinge elements and the door hinge elements to each other by simply inserting the respective articulation pins in their seats, with no need of further operations for adjusting and recording the geometrical settings, either along the line for completion of the vehicle body prior to painting, or along the final vehicle assembly line. - detecting a plurality of dimensions measured along coordinates X, Y, Z on the vehicle body and on the doors, with the aid of proper references, for qualifying the geometric conditions of the coupling between the doors and the respective door receiving seats, - determining, on the basis of the detected dimensions, the mounting positions of each pair of hinge elements (CF,CM) to be connected separately to the vehicle body sides and to the door structures, so as to obtain that the pivots for support and articulation of the doors are well aligned with each other and that the geometrical setting of the door is consistent with the respective door receiving seats on the vehicle body sides, according to the assembly project drawing, - mounting the aforementioned hinge elements (CF,CM) on the body in the positions thus determined, - providing the doors with the respective door hinge elements, while making - via software - the system for mounting the door hinge elements to interact with the system for mounting the body hinge elements, in order to substantially ensure consistency with the assembly project drawing, - mounting the doors on the vehicle body by connecting the body side hinge elements and the door hinge elements to each other by simply inserting the respective articulation pins in their seats, with no need of further operations for adjusting and recording the geometrical settings, either along the line for completion of the vehicle body prior to painting, or along the final vehicle assembly line.