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The modified pressure with stored equalisation (MPSE) method of soundfield equalisation is universally recommended for use in clinical practice during the real-ear measurement (REM) verification of open-canal (OC) hearing aids. This is because the MPSE method deactivates the reference microphone for aided measurements and so is not susceptible to the leakage of amplified sound from the open ear canal when fitting OC devices. However, the deactivation of the reference microphone means that the MPSE method will be susceptible to errors resulting from head movements during aided measurements and the magnitude of such errors has not been well explored in the literature. Therefore, this study aimed to investigate how much small horizontal head movements can influence the accuracy of REMs when using the MPSE method. Real-ear unaided responses (REURs) were measured in 28 participants at horizontal head deviations of 0°, ±5°, ±10°, ±15°, and ±20° azimuth relative to the two loudspeaker-to-client azimuths of 0° and 45°. The difference between the baseline REUR measured when no head deviation was made and the REURs measured for each head deviation was then calculated so that the magnitude of REM errors arising from these head movements could be determined. Statistical analysis revealed no significant difference between the REM errors for the 0° and 45° loudspeaker-to-client azimuths. REM errors of typically less than 1 dB were obtained when no head deviation was made (i.e. 0° head deviation), indicating that no clinically significant errors are introduced when using the MPSE method provided patients keep their heads still. However, the REM errors were found to increase with increasing head deviation up to ±20° where the errors were typically less than 2 dB. The magnitude of these head movement induced errors are not clinically significant on their own but it is recommended that head movements are minimised as much as possible so that REM targets can be matched more accurately.