The 2-year efficacy and safety of idebenone were studied in a prospective, randomized, double-blind multicentre study in 3 parallel groups of patients with dementia of the Alzheimer type (DAT) of mild to moderate degree. A total of 450 patients were randomized to either placebo for 12 months, followed by idebenone 90 mg tid for another 12 months (n = 153) or idebenone 90 mg tid for 24 months (n = 148) or 120 mg tid for 24 months (n = 149). The primary outcome measure was the total score of the Alzheimer's Disease Assessment Scale (ADAS-Total) at month 6. Secondary outcome measures were the ADAS cognitive (ADAS-Cog) and noncognitive score (ADAS-Noncog), the clinical global response (CGI-Improvement), the SKT neuropsychological test battery, and the Nurses' Observation Scale for Geriatric Patients (NOSGER-Total and IADL subscale). Safety parameters were adverse events, vital signs, ECG and clinical laboratory parameters. During the placebo controlled period (the first year of treatment), idebenone showed statistically significant dose-dependent improvement in the primary efficacy variable ADAS-Total and in all the secondary efficacy variables. There was no evidence for a loss of efficacy during the second year of treatment, as a further improvement of most efficacy variables was found in the second year in comparison to the results at the 12 months visit. Also, a clear dose effect relationship (placebo/90 mg < idebenone 90 mg < idebenone 120 mg) was maintained throughout the second year of treatment. This suggests that idebenone exerts its beneficial therapeutic effects on the course of the disease by slowing down its progression. Safety and tolerability of idebenone were good and similar to placebo during the first year of treatment and did not change during the second year.