A method of performing a real ear measurement in a hearing assistance device comprising an ITE part adapted for being located at or in an ear canal of a user, the hearing assistance device comprising a forward path defined between a measurement input transducer for converting an input sound signal to an electric input signal, an output transducer for converting an electric output signal to an output sound, a feedback cancellation system comprising an adaptive feedback estimation unit in the form of an adaptive filter estimating an acoustic feedback path from the output transducer to the measurement input transducer, the feedback estimation unit for estimating the acoustic feedback path provides first and second impulse responses of a first and a second controlled acoustic feedback path, a memory for storing one or more acoustic feedback estimates, the forward path comprising a processing unit operatively connected to the memory, and a probe signal generator for generating a probe signal, the probe signal generator being operatively connected to the output transducer, at least in a specific probe signal mode, in which probe signal mode the processing unit of the forward path is disabled and said probe signal generator is enabled to play a probe signal via said output transducer, the method comprising the steps of a1) providing a first controlled acoustic feedback path from the output transducer to the measurement input transducer via a standard acoustic coupler; b1) generating a first probe signal; c1) estimating and storing a first estimate of the first controlled acoustic feedback path by finding the filter coefficients of said adaptive filter which minimizes a prediction error between said first probe signal and the measurement input transducer signal of the first controlled acoustic feedback path; and a2) providing a probe tube inserted into the ear canal next to the ear mould establishing said second controlled acoustic feedback path from the output transducer to the measurement input transducer via the residual volume between the ITE part of the hearing aid device and the user's eardrum; b2) generating a second probe signal; c2) estimating and storing a second estimate of the second controlled acoustic feedback path by finding the coefficients of an adaptive filter which minimizes a prediction error between said second probe signal and the measurement input transducer signal of the second controlled acoustic feedback path; and e) determining a real ear to coupler difference from said impulse response of said first and second acoustic feedback estimates. a1) providing a first controlled acoustic feedback path from the output transducer to the measurement input transducer via a standard acoustic coupler; b1) generating a first probe signal; c1) estimating and storing a first estimate of the first controlled acoustic feedback path by finding the filter coefficients of said adaptive filter which minimizes a prediction error between said first probe signal and the measurement input transducer signal of the first controlled acoustic feedback path; and a2) providing a probe tube inserted into the ear canal next to the ear mould establishing said second controlled acoustic feedback path from the output transducer to the measurement input transducer via the residual volume between the ITE part of the hearing aid device and the user's eardrum; b2) generating a second probe signal; c2) estimating and storing a second estimate of the second controlled acoustic feedback path by finding the coefficients of an adaptive filter which minimizes a prediction error between said second probe signal and the measurement input transducer signal of the second controlled acoustic feedback path; and e) determining a real ear to coupler difference from said impulse response of said first and second acoustic feedback estimates.