Amyotrophic lateral sclerosis (ALS) is a rare disease involving selective and progressive degeneration and disappearance of motor neurons. Oxidative stress is believed to contribute to its pathogenesis. We have investigated the efficacy and safety of edaravone, a free radical scavenger previously approved for treatment of acute cerebral infarction, in ALS patients. Within an open trial design, 20 subjects with ALS received either 30 mg (5 subjects) or 60 mg (15 subjects) of edaravone via intravenous drip once per day. Two weeks of administration was followed by a two-week observation period. This four-week cycle was repeated six times. The primary endpoint was the change in the revised ALS functional rating scale (ALSFRS-R) score, while the secondary endpoint was 3-nitrotyrosine (3NT) level in cerebrospinal fluid (CSF). Efficacy was evaluated in the 60 mg group. During the six-month treatment period, the decline in the ALSFRS-R score (2.3+/-3.6 points) was significantly less than that in the six months prior to edaravone administration (4.7+/-2.1 points); the difference between the two was 2.4+/-3.5 points (Wilcoxon signed rank test, p = 0.039). In almost all patients, CSF 3NT, a marker for oxidative stress, was markedly reduced to almost undetectable levels at the end of the six-month treatment period. Data from the present study suggest that edaravone is safe and may delay the progression of functional motor disturbances by reducing oxidative stress in ALS patients.