The Japanese Pharmacopoeia (JP) permits the use of water prepared by water processing systems based on reverse osmosis (RO) and/or ultrafiltration (UF) membranes (membrane-processed water) for Water for Injection (WFI). However, European Pharmacopoeia (EP) permits only distilled water for WFI, and the United States Pharmacopeia (USP) specifies that WFI is to be produced by distillation or a purification process that is equivalent or superior to distillation in the removal of chemicals and microorganisms. The reason why these Pharmacopoeias deal with membrane-processed water differently is because of remaining concerns on the reliability of membrane-processed water. This study intends to establish and confirm the reliability of membrane-processed water for WFI through the collection and evaluation of long-term operational data of RO and/or UF membrane-based pharmaceutical water processing systems in Japan. In 2005, members of this study group visited 10 Japanese pharmaceutical plants that have been preparing purified water (PF) and WFI for pharmaceutical use with membrane-based water processing systems. The group investigated operational data of the systems for the past several years, and evaluated whether such systems have been producing water suitable as WFI consistently and reliably. Based on the result of the evaluation of data obtained from 10 visited plants, it was concluded that all of the membrane-based water processing systems were designed, operated, and properly controlled such that water suitable as WFI was consistently achieved with the entire system (including the pretreatment units).