A new apparatus is developed to measure the Water vapor permeability resistance of textiles with and without temperature differences imposed on both sides of a specimen. Water vapor resistance is measured for a combination of temperature and pressure that simulates elevated altitudes. The effect of temperature on water vapor resistance is small. while that of pressure is significant, that is, water vapor resistance decreases with increasing simulated altitude due to an increase in the water vapor diffusion coefficient with increasing altitude. The amount of condensation in the specimen tends to increase with increasing simulated altitude. Moreover, water vapor resistance decreases apprecia bly due to increased condensation in the specimen. In addition, the reduced resistance further enhances the amount of condensation. These results indicate that decreased water vapor resistance enhances condensation in clothes and thus may cause further discomfort and a drop in body temperature at high altitudes.