Positron emission tomography of the brain following intravenous injection of (+) (R) and (-) (S) N-[11C-methyl]nicotine showed a marked reduced uptake of both isomers, especially the (R) form, in Alzheimer patients as compared to age-matched controls. The significantly larger difference between the uptake values of the (S)- and (R)-enantiomers of 11C-nicotine in Azheimer brains may be of diagnostic value.