Owing to the pharmacokinetic properties of physostigmine when administered by conventional routes, long-term cholinergic treatment of Alzheimer's disease is difficult to manage. In order to overcome the problems associated with the oral and intravenous application of physostigmine, and to improve patients' compliance, a transdermal therapeutic system was developed. The efficacy and tolerability of this system were evaluated in a double-blind, randomized, multicenter study comparing patches containing 30 mg and 60 mg physostigmine with a placebo patch. The clinical trial followed the basic principles of the various guidelines on the evaluation of anti-dementia drugs, and included patients with mild to moderate probable Alzheimer's disease. A total of 204 patients with probable Alzheimer's disease were included in the study. Of these, 136 patients were eligible for the according-to-protocol analysis of efficacy, 167 subjects for the intention-to-treat analysis of efficacy, and 181 patients were included in the safety analysis. In contrast to the hypothesis to be tested, the efficacy of physostigmine was not superior to that of placebo after a treatment period of 24 weeks. On the contrary, there was even a slight, but not statistically significant, trend toward a better outcome in the placebo group. Median physostigmine plasma concentrations of approximately 100 pg/ml were measured, showing a high degree of interindividual variability and no linear dose relationship between the 30 mg and 60 mg dosages. Plasma cholinesterase activity was not significantly affected by physostigmine. The physostigmine patch application in doses of 30 mg and 60 mg apparently did not lead to physostigmine plasma concentrations that were sufficient to compensate for cholinergic deficiencies in affected brain areas and produce clinical benefits. Both the drug and the transdermal system were generally well tolerated under the study conditions. Modifications of the patch system may perhaps make it possible to achieve higher physostigmine plasma concentrations, which seem to be required to induce the expected beneficial effects during long-term treatment of Alzheimer's disease.