C9orf72is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) in Caucasian populations. However, the relationship betweenC9orf72repeats and Alzheimer’s disease (AD) was not clear. Additionally, there were few articles assessingC9orf72in other ethnicities with ALS. In this meta-analysis, we aimed to investigate the relationship betweenC9orf72repeat expansions (≥30 repeats) and intermediate repeat copies (20–29 repeats) and AD or ALS. The results suggested positive correlations betweenC9orf72repeat expansions and the risk of Alzheimer’s disease (OR = 6.36, 95% CI = 3.13–12.92, andp<0.00001), while intermediate repeat copies ofC9orf72gene were not associated with the risk of the disease.C9orf72repeat expansions were positively correlated with the risk of familial and sporadic ALS (OR = 293.25, 95% CI = 148.17–580.38, andp<0.00001; OR = 35.57, 95% CI = 19.61–64.51, andp<0.00001). There was a positive correlation between the gene variations and ALS risk among Caucasians and Asians (OR = 57.56, 95% CI = 36.73–90.22, andp<0.00001; OR = 6.35, 95% CI = 1.39–29.02, andp=0.02).