OBJECTIVE: Because previous studies have demonstrated that patients with chronic schizophrenia have deficits in mismatch negativity associated with poor everyday functioning, the authors investigated the longitudinal stability of mismatch negativity deficits and their longitudinal relationship to poor functional status. METHOD: Ten patients with chronic schizophrenia underwent testing twice over a 1–2-year period. Tests used included mismatch negativity, the Scale for the Assessment of Negative Symptoms, the Scale for the Assessment of Positive Symptoms, and the Global Assessment of Functioning Scale. Patients’ test results from both sessions were compared with results for 10 age-matched normal subjects. RESULTS: Patients with schizophrenia had mismatch negativity deficits with large effect sizes that were reliable over time. Mismatch negativity deficits were also significantly associated with poor functional status at both the first and second session. CONCLUSIONS: Mismatch negativity deficits and their relationship to poor functional status are stable over time in patients with chronic schizophrenia, suggesting that mismatch negativity may be useful for assessing medication response and other factors in longitudinal studies. Future studies will clarify the emergence, progression, and treatment effects of mismatch negativity deficits in patients with schizophrenia.