Evidence showed impaired cognitive ability of patients with pituitary adenoma as an intracranial tumor. In the present study, we investigated the preattentive processing of emotional faces in pituitary adenoma patients by recording and analyzing the visual mismatch negativity elicited by deviant emotional faces. In the normal control group, deviant emotional faces elicited expression-related mismatch negativity (EMMN), with a more pronounced negative waveform for sad than happy face conditions. Compared with the normal group, EMMN related to sad faces was smaller in patients, whereas the EMMN related to happy faces remained the same. These data indicate the functional dysfunction of negative emotional processing at the preattentive stage of information processing in pituitary adenoma patients.