Objectives: In patients with bilateral asymmetrical hearing loss (AHL) where only one hearing aid is available, it is difficult to decide which ear to amplify. The aims of this study were to evaluate the outcomes of hearing-aid use for AHL patients fitted with a hearing aid in their worse ear only. Design: Retrospective study. Setting: Secondary-referral, university-based hospital. Participants: 102 adults with asymmetrical mixed or sensorineural hearing loss. AHL was classified into three subgroups: unilateral hearing loss (UHL), and asymmetrical hearing loss type 1 (AHL1) and type 2 (AHL2). Main outcome measures: (1) The time for wearing a hearing aid, (2) the hearing in noise test (HINT)), (3) sound localization test, and (4) the Korean version of the International Outcome Inventory for Hearing Aids (IOI-HA). Results: The 1 kHz-hearing threshold of the better ear was significantly better in the intermittent users than in the successful users for UHL. Among the etiologies of AHL, sudden hearing loss was associated with significantly better outcomes of hearing-aid use for AHL, UHL, and AHL1 patients. Younger age was associated with significantly better outcomes for AHL and AHL2. Conclusions: The results indicate the importance of good hearing at 1 kHz in the better ear when counseling UHL patients who will receive a monaural hearing aid in their worse ear. We also suggest that clinicians should recommend a monaural hearing aid for the worse ear to younger patients with AHL, especially in cases of sudden hearing loss.