Animal and pathological studies suggest that inflammation may contribute to amyotrophic lateral sclerosis (ALS) pathology and that non-steroidal anti-inflammatory drugs (NSAIDs) might be protective. However, there are no prospective data on the relation between NSAID use and ALS risk in humans. The relation between NSAID use and ALS risk was explored in five large prospective cohort studies (the Nurses' Health Study, the Health Professionals Follow-up Study, the Cancer Prevention Study II Nutrition Cohort, the Multiethnic Cohort Study, and the National Institutes of Health - AARP Diet and Health Study). Detailed NSAID information was sought from 780,000 participants, 708 of whom developed ALS during follow-up. Cox proportional hazards models were used within each cohort and cohort-specific estimates were pooled with random effects models. Results showed that neither non-aspirin NSAID use, nor aspirin use was associated with ALS risk overall. The multivariable, pooled relative risk was 0.96 (95% CI 0.76-1.22) among non-aspirin NSAID users compared with non-users. Duration of NSAID use in years and frequency of NSAID use were not associated with ALS risk overall. In conclusion, the results do not support an overall effect of NSAIDs on ALS risk, but because NSAIDs have heterogeneous effects, a role of individual compounds cannot be excluded.