It has been reported that patients with pure conductive hearing loss (CHL) have remarked that their hearing is better in the presence of background noise. This study investigated the ability for speech discrimination under background noise in adult subjects with CHL, using the Mandarin Hearing in Noise Test (M-HINT).Seventeen subjects with unilateral CHL and 15 with bilateral CHL participated in this study. Twenty normal-hearing subjects served as controls. During the M-HINT, the sentences and noise were presented in a soundproof chamber. Reception threshold for sentences (RTS) in quiet were obtained first, and then three more blocks, including noise from the front, right and left, were measured for each subject.The RTS in quiet was significantly elevated in unilateral and bilateral CHL groups. For the unilateral group, regardless of whether noise came from the front, affected side, or normal side, the signal-tonoise ratio (SNR) that was needed to reach 50% correction was significantly higher than in the control group. For the bilateral group, the SNR for noise from the front, left and right was significantly elevated compared with that in the unilateral and control groups. The noise composite score was also significantly different among these three groups (control < unilateral < bilateral).There was reduced speech discrimination ability under background noise in adult subjects with unilateral or bilateral CHL.