Background The early diagnosis of Alzheimer's disease (AD) may help reduce disability, enhance quality of life, and aid clinical trials. Portions of olfactory cortex are the initial sites of AD pathology and patients with AD often have more degeneration of their left than right hemisphere. Since the olfactory epithelium projects mainly to the ipsilateral olfactory cortex, patients with AD may demonstrate an asymmetrical (left greater than right) decrement of odor detection sensitivity. This retrospective, case-control study assessed a quick olfactory test that may help diagnose AD. Methods Participants with probable AD (N = 18), mild cognitive impairment (MCI, N = 24), other causes of dementia (OD, N = 26) and matched controls (OC, N = 26) were tested, with closed eyes, for their ability to detect an odor, one nostril at a time. A container of 14 g of peanut butter was opened, held medially at the bottom of a 30 cm ruler, and moved up 1 cm at a time during the participants' exhale. Upon odor detection, the distance between the subject's nostril and container was measured. Results The mean odor detection distance of AD patients' left nostril (5.1 cm), and not their right (17.4 cm), was significantly less (F(3,90) = 22.28, p < 0.0001) than the other groups. The mean, standard error, and 95% Confidence Interval of the L-R nostril odor detection difference (cm) for AD were - 12.4 ± 0.5, (- 15.0,- 9.8); for MCI were - 1.9 ± 1.2, (- 4.2,0.4); for OD were 4.8 ± 1.0, (2.6,6.9); and for OC were 0.0 ± 1.4 (- 2.2,2.1). Conclusion This non-invasive and inexpensive left-right nostril odor detection test appears to be a sensitive and specific test for probable AD. © 2013 Elsevier B.V.