In a cross-sectional study of outpatients diagnosed with dementia of the Alzheimer type who had been treated with a broad variety of drugs supposed to improve cognition or to delay cognitive decline, we have investigated the effects of abruptly discontinuing therapy on cognition. Termination of therapy with any cholinesterase inhibitor was associated with a cognitive decline during the following 6-7 weeks which was significantly more pronounced than that experienced by patients who had received nootropic drugs or calcium channel blockers (3.41 vs. 1.17 points on the ADAS-Cog scale; -1.14 vs. -0.06 points on the MMSE scale). This effect was not modified by gender, apolipoprotein E genotype, or the extent of ventricular enlargement on CT scans. Its magnitude was comparable to the cognitive response observed in published clinical trials when cholinesterase therapy commenced, and also with the data obtained during a 6-week placebo washout phase.