The present study examined the ability of the Mini-Mental State Examination (MMSE), a cognitive screening test commonly employed in research and clinical applications, to predict level of performance on a comprehensive neuropsychological battery, the Luria-Nebraska Neuropsychological Battery (LNNB). The tests were administered to a diagnostically mixed sample of 90 psychiatric inpatients. Correlations between the tests were statistically significant but modest and the MMSE was not able to detect many patients who demonstrated significant deficits on the LNNB. Additional correlational analyses between the MMSE and the Wechsler Adult Intelligence Scale-Revised in a subset of patients (n = 72) also showed a modest relationship between these instruments. The results suggest that the MMSE may seriously underestimate cognitive impairment in samples exclusively composed of psychiatric patients, making it inappropriate to rely solely on this instrument to distinguish psychiatric patients with and without cognitive deficit.