The efficacy and safety of eptastigmine in patients with probable Alzheimer disease was evaluated in a double-blind, placebo-controlled, parallel group study. Patients with mild to moderate dementia were randomly assigned to placebo, eptastigmine 10 mg three times a day (t.i.d.), or eptastigmine 15 mg t.i.d. over 25 weeks. The Alzheimer Disease Assessment Cognitive Subscale (ADAS-Cog) and the Clinician's Interview-Based Impression of Change Plus (CIBIC-Plus) were the primary outcome measures for efficacy. Twenty-six centers recruited 320 patients: 106 on placebo, 105 on eptastigmine 10 mg t.i.d., and 109 on eptastigmine 15 mg t.i.d. Six patients on placebo (6%), 18 patients on eptastigmine 10 mg t.i.d. (17%), and 10 patients on eptastigmine 15 mg t.i.d. (9%) discontinued study treatment. The intent-to-treat analysis on 315 patients showed a statistically significant (p=0.047) difference of 2.0 points on ADAS-Cog between the placebo and the eptastigmine 15 mg t.i.d. group at the end of treatment. Patients in the 10 mg t.i.d. group performed better than did placebo-treated patients on the Spontaneous Behavior Interview (SBI) total scores (p=0.015) and on the Activities of Daily Living (ADL, p=0.043) and Behavioral Problems (BP, p=0.028) subscales. The differences in favor of the eptastigmine groups on the CIBIC-Plus did not reach statistical significance. In a post hoc subgroup analysis by staging, the effect size of eptastigmine was found to be greater in the most severely impaired patients (Global Deterioration Scale rating of 4 and 5 at screening) reaching statistical significance in both ADAS-Cog (p=0.007) and CIBIC-Plus (p=0.038). In this patient subgroup (n=222), there was also a significant effect of eptastigmine on SBI (p=0.019). The drug was generally well tolerated, with 8% of patients withdrawing due to adverse events versus 5% on placebo. Adverse events were recorded in 35 patients (33%) on placebo compared with 41 (39%) on eptastigmine 10 mg t.i.d. and 38 (35%) on eptastigmine 15 mg t.i.d. This study shows that eptastigmine doses up to 15 mg t.i.d. for 25 weeks are well tolerated. The drug positively affects cognitive performance of Alzheimer patients. This effect appears greater in more severely impaired patients and also impacts on their behavioral performance.