No previous study has investigated which ear should be fitted with a hearing aid. Fifty-eight patients with bilateral hearing loss of 25-75 dB HL were sequentially fitted with an ear level hearing aid for 10 weeks in each ear. Patients were then asked their preference for side of fitting, and their reasons for this preference. Thirty-five patients had symmetrical hearing: 17 (49%) preferred one ear because they could hear better with the aid in that ear, and 12 (34%) chose one side for practical reasons. Twenty-three patients had asymmetrical hearing: 8 (35%) preferred one side for practical reasons, and 13 (57%) could hear better with the aid in one ear, which was invariably the poorer hearing ear. As the preferred ear cannot be reliably predicted, bilateral ear moulds should be provided. If only one mould is provided, it should be for the poorer hearing ear.