The net water drag coefficient ( α NWD ) is supposedly an essential index in water management, and must be negative for system operation for polymer electrolyte fuel cell (PEFC) systems. In this study, α NWD is carefully examined by measurement of the water content in the gases supplied to and those discharged from the cell/stack under gas supply conditions similar to those in a real PEFC system. We focus on the effect of anode hydrogen (H2) flow conditions (i.e., flow rate and relative humidity) on α NWD under a constant flow condition of cathode air. Measured characteristics of current density ( i )- α NWD in which the relative humidity of discharged H2 from the anode is constant at around 100%, show that α NWD is stable when i exceeds a certain threshold. The value of α NWD in this stable region is particularly sensitive to changes in the stoichiometric ratio of H2. These measurements also suggest that in the α NWD stable region, almost the entire area of the membrane is fully hydrated, and the effect of back diffusion on α NWD becomes negligible. Analysis of the observed relationships between α NWD and several variables indicate that increasing the H2 circulation flow rate effectively prevents anode dry-out.