Objective: To describe clinical features and diagnostic tests of the MM2 cortical subtype in sporadic Creutzfeldt- Jakob disease. Methods: Clinical symptoms, magnetic resonance imaging studies, electroencephalograms, and cerebrospinal fluid markers were studied in 12 patients with genetically and neuropathologically verified sporadic Creutzfeldt- Jakob disease. Histological findings were semi- quantitatively evaluated. Results: Compared with classical sporadic Creutzfeldt- Jakob disease, the disease duration was prolonged (median, 14 months). All patients had dementia and early and prominent neuropsychological signs such as spatial disorientation, aphasia, or apraxia. Alzheimer disease was the most frequent initial diagnosis (33% ). Increased S100B protein in the cerebrospinal fluid was found in 100% ; the 14- 3- 3 protein test was positive in 91% . Electroencephalograms revealed periodic sharp wave complexes in 42% . T2- weighted magnetic resonance imaging showed basal ganglia hyperintensities in only 1 patient, and cortical hyperintensities were not necessarily present. Severe cortical damage was the most prominent histological feature. Conclusions: The S100B (100% ) and 14- 3- 3 (91% ) protein investigations were the most sensitive diagnostic tests. Prolonged disease duration, dementia as the only typical Creutzfeldt- Jakob disease symptom for a longer time, and low sensitivity of magnetic resonance imaging studies and electroencephalograms make the diagnosis in the MM2 cortical subtype difficult. Therefore, detailed clinical investigation is especially important in this sporadic Creutzfeldt- Jakob disease subtype. We suggest that rapidly progressive dementia with early and prominent neuropsychological deficits in older patients should lead to suspicion of the MM2 cortical subtype even if other neurological deficits are absent. At least some cases of MM2 cortical sporadic Creutzfeldt- Jakob disease may be misdiagnosed as rapidly progressive Alzheimer disease. Objective: To describe clinical features and diagnostic tests of the MM2 cortical subtype in sporadic Creutzfeldt-Jakob disease. Methods: Clinical symptoms, magnetic resonance imaging studies, electroencephalograms, and cerebrospinal fluid markers were studied in 12 patients with genetically and neuropathologically verified sporadic Creutzfeldt- Results: Compared with classical sporadic Creutzfeldt-Jakob disease, the disease duration was prolonged (median, 14 months). All patients had dementia and early and prominent neuropsychological signs such as spatial disorientation, aphasia , or apraxia. Alzheimer disease was the most frequent initial diagnosis (33%). Increased S100B protein in the cerebrospinal fluid was found in 100%; the 14- 3- 3 protein test was positive in 91%. Electroencephalograms revealed periodic sharp wave complexes in 42%. T2-weighted magnetic resonance imaging showed basal ganglia hyperin Tensions in only 1 patient, and cortical hyperintensities were not necessarily present tests. Severe cortical damage was the most prominent histological feature. Conclusions: The S100B (100%) and 14- 3- 3 (91%) protein investigations were the most sensitive diagnostic tests Prolonged disease duration, dementia as the only typical Creutzfeldt-Jakob disease symptom for a longer time, and low sensitivity of magnetic resonance imaging studies and electroencephalograms make the diagnosis in the MM2 cortical subtype difficult. Therefore, detailed clinical investigation is especially important in this sporadic Creutzfeldt- Jakob disease subtype. We suggest that rapidly progressive dementia with early and prominent neuropsychological deficits in older patients should lead to suspicion of the MM2 cortical subtype even if other neurological deficits are absent. At least some cases of MM2 cortical sporadic Creutzfeldt- Jakob disease may be misdiagnosed as rapidly progressive Alzheimer disease.