BACKGROUND: Acquired hearing loss is common and its incidence increases markedly with age. In most people, 'age-related' hearing loss is sensorineural (due to the loss of cochlear hair cells) and bilateral, affecting both ears to the same degree. Hearing loss categorised as mild, moderate or severe is primarily managed with hearing aids. People with bilateral hearing loss may be offered one aid, fitted to one specific ear, or two aids fitted to both ears. There is uncertainty about the relative benefits to people with hearing loss of these different strategies. OBJECTIVES: To assess the effects of bilateral versus unilateral hearing aids in adults with a bilateral hearing impairment. SEARCH METHODS: The Cochrane ENT Information Specialist searched the ENT Trials Register; Cochrane Register of Studies Online; PubMed; Ovid Embase; CINAHL; Web of Science; ClinicalTrials.gov; ICTRP and additional sources for published and unpublished trials. The date of the search was 8 June 2017. SELECTION CRITERIA: Randomised controlled trials (RCTs) comparing the fitting of two versus one ear-level acoustic hearing aids in adults (over 18 years) with a bilateral hearing impairment, both ears being eligible for hearing aids. DATA COLLECTION AND ANALYSIS: We used the standard methodological procedures expected by Cochrane. Our primary outcomes were patient preference for bilateral or unilateral aids, hearing-specific health-related quality of life and adverse effects (pain or discomfort in the ear, initiation or exacerbation of middle or outer ear infection). Secondary outcomes included: usage of hearing aids (as measured by, for example, data logging or battery consumption), generic health-related quality of life, listening ability and audiometric benefit measured as binaural loudness summation. We used GRADE to assess the quality of the evidence for each outcome; this is indicated in italics. MAIN RESULTS: We included four cross-over RCTs with a total of 209 participants, ranging in age from 23 to 85 and with a preponderance of men. All the studies allowed the use of hearing aids for a total period of at least eight weeks before questions on preference were asked. All studies recruited patients with bilateral hearing loss but there was considerable variation in the types and degree of sensorineural hearing loss that the participants were experiencing.Three of the studies were published before the mid-1990s whereas the fourth study was published in 2011. Therefore, only the most recent study used hearing aids incorporating technology comparable to that currently readily available in high-income settings. Of the four studies, two were conducted in the UK in National Health Service (NHS - public sector) patients: one recruited patients from primary care with hearing loss detected by a screening programme whereas the other recruited patients who had been referred by their primary care practitioner to an otolaryngology department for hearing aids. The other two studies were conducted in the United States: one study recruited only military personnel or veterans with noise-induced hearing loss whereas about half of the participants in the other study were veterans.Only one primary outcome (patient preference) was reported in all studies. The percentage of patients who preferred bilateral hearing aids varied between studies: this was 54% (51 out of 94 participants), 39% (22 out of 56), 55% (16 out of 29) and 77% (23 out of 30), respectively. We have not combined the data from these four studies. The evidence for this outcome is of very low quality.The other outcomes of interest were not reported in the included studies. AUTHORS' CONCLUSIONS: This review identified only four studies comparing the use of one hearing aid with two. The studies were small and included participants of widely varying ages. There was also considerable variation in the types and degree of sensorineural hearing loss that the participants were experiencing.For the most part, the types of hearing aid evaluated would now be regarded, in high-income settings, as 'old technology', with only one study looking at 'modern' digital aids. However, the relevance of this is uncertain, as this review did not evaluate the differences in outcomes between the different types of technology.We were unable to pool data from the four studies and the very low quality of the evidence leads us to conclude that we do not know if people with hearing loss have a preference for one aid or two. Similarly, we do not know if hearing-specific health-related quality of life, or any of our other outcomes, are better with bilateral or unilateral aids.