An adapter for coupling a probe tube for real ear measurements to a microphone of a hearing aid. The adapter comprises a sleeve adapted to be arranged around the housing of a hearing aid. The sleeve comprises a sound opening allowing sound to pass from the probe tube to the microphone. The adapter may form integral part of a device for real ear measurements, comprising a probe tube and an adapter for connecting the probe tube to a microphone, where the adapter comprises the sleeve adapted to be placed around the housing of a hearing aid said sleeve comprising a sound opening allowing sound to pass from the probe tube to the microphone. The device may be used in a method for performing real ear measurements comprising the steps of providing the device, fitting said device around the housing of the hearing aid so that the sound opening is placed over the built-in microphone of the hearing aid, placing the probe tube in the ear in such a way that a free end thereof presents an opening in the cavity between the earplug and the tympanic membrane, detecting the sound pressure from sounds produced by said hearing aid in said cavity using said microphone.