During 9-20 Nov 2011, four prisoners in the national prison died with unknown cause and many prisoners developed diarrhea and neuropathy. The Bureau of Epidemiology launched an investigation to identify etiology and source of outbreak, and implement control measures. A case was defined as a prisoner or guard who developed gastrointestinal (GI) symptoms or neurological symptoms during 1 Sep to 31 Dec 2011. Foods, water, blood and urine samples were tested for heavy metals, vitamins B1 and B12. The prison had 33 guards, 3,668 male prisoners and 555 female prisoners. Among 475 prisoners who met the case definition, 307 (64.6%) GI cases, 49 (10.3%) neurological cases and 119 (25.1%) cases with both GI and neurological symptoms were identified. No case was found among the guards. Attack rates among male and female prisoners were 12.6% (462/3,668) and 2.3% (13/555) respectively. No female prisoners developed neuropathy. Eleven male prisoners had severe distal symmetrical peripheral neuropathy. Four men aged 26-47 years died after developing acute dyspnea. The cases rapidly increased after pipeline of the prison damaged on 2 Nov 2011. Blood and urine samples illustrated vitamin B1 deficiency and high arsenic concentration respectively. This outbreak of peripheral neuropathy possibly resulted from arsenic contaminated drinking water. After providing clean water on 24 Nov 2011, the outbreak subsided within 10 days.