The identification of cognitive impairment in general practice requires short but accurate tests. For epidemiologic surveys and genetic family studies cognitive tests are desirable which can be administered over the telephone. We assessed the ability of a telephone version of the Modified Mini Mental State Examination (T3MS) to identify mild cognitive impairment (MCI) and mild dementia in Alzheimer's disease (AD) and compared it with the diagnostic accuracy of the conventional Mini Mental State Examination (MMSE). The study refers to 34 patients of the outpatient clinic for cognitive disorders of the technical university of Munich of whom 18 had MCI and 16 had mild dementia in AD, respectively. The study also included 14 cognitively unimpaired age-matched probands. The T3MS and MMST were validated against an expert diagnosis base on a comprehensive diagnostic workup. Statistical analysis was performed using the receiver-operator-characteristics (ROC) method. The T3MS outperformed the MMST in the distinction between MCI patients and cognitively unimpaired individuals. In the separation between cognitively unimpaired probands and patients with mild AD the T3MS achieved a sensitivity and specificity of 100 %. The T3MS is a short and practical but accurate telephone test for the identification of mild dementia in AD for use in epidemiological surveys and genetic family studies. The interview achieves higher diagnostic precision than the MMSE and contributes to a valid assessment of cognitive performance. For the identification of mild cognitive impairment, however, the T3MS was less appropriate.