To evaluate the relationship between cognitive status and incident delirium, 291 geriatric patients on internal medicine wards were evaluated on admission with the Mini-Mental State Examination (MMSE) and Confusion Assessment Method-Spanish. Those with incident delirium were assessed using the Delirium Rating Scale-Revised-98 (DRS-R98). Delirium incidence was 11.7%, and 82 patients (28.2%) had cognitive deficits on MMSE. As cognitive impairment worsened, the risk for delirium increased linearly, and for each unit of MMSE worsening the DRS-R98 severity score worsened 0.4 points (F=5.39, df=1, p=0.027). Optimal MMSE cutoff score from receiver-operating characteristic curve analysis was 24.5. Even mild cognitive deficits increase delirium risk and severity.