The present invention relates to a method of adjusting at least one defective, main or anti-torque rotor of a particular rotorcraft. The method uses a neural network representing the relationships between firstly accelerations representative of vibration generated on at least a portion of a reference rotorcraft, and secondly defects and adjustment parameters. After determining the defects, if any, of a defective rotor, an adjustment value .alpha. is defined for at least one of the adjustment parameters, advantageously by minimizing the following relationship:.times..times..lamda..times..times..times..lamda..times..tim- es. .times..times..lamda..times..function..alpha..gamma. .lamda..function..alpha..gamma..gamma. ##EQU00001##