Our objective is to describe the results of a phase II/III, 12-months, double-blinded, single-centre, randomized, parallel (1:1), clinical trial performed to evaluate the efficacy and safety of memantine in ALS. Patients with probable or definite ALS of less than 36 months disease duration and progression over a one-month lead-in period were randomly assigned to placebo or memantine at 20 mg/day. The primary endpoint was 12-months ALSFRS decline. Forced vital capacity, manual muscle testing, visual analogue scale, quality of life, motor unit number estimation and neurophysiological index were the secondary endpoints. The number of patients included was based on the assumption of a 50% change in the ALSFRS decline. Safety and adverse events were evaluated. Sixty-three patients were included in the trial. Memantine did not show more adverse events or laboratory changes than placebo. Primary and secondary outcomes were not different between groups by intention-to-treat and per-protocol analysis. The most sensitive measurements were neurophysiological, which declined linearly over time. In conclusion, the results of this study show that memantine is well tolerated and safe in ALS patients. We did not observe any evidence of efficacy for memantine but we cannot exclude a positive outcome on survival.