Rose plants (Rosa hybrida L. 'Asami Red') were grown hydroponically at 25°C in a 12-h day and 20°C in 12-h night cycle. The preharvest relative humidity (RH) was set at constant 60% or 85% for a constant humidity experiment, and 60 (day)/85 (night) % or 85 (day)/60 (night) % for a altered humidity experiment. The cut flower yield was greater at the constant high RH condition of 85% than at the constant low RH of 60%. Cut flowers grown at 85% RH had larger stomata in the dark than those at 60% RH. The cut roses grown at 85% showed high stomatal conductance even in the dark and poorly responded to light-dark alternation, resulting in increased transpiration and consequent decreased vase life. In altered humidity experiment, the cut flower yield was higher at 85/60% RH than at 60/85% RH. Stomatal size, responses of stomata to light-dark alternation, daily transpiration and vase life of cut flowers did not differ significantly between these two RH treatments and were almost similar to those of the roses grown at constant 60% RH. Cyclic control of day-night RH is an effective means to achieve both high yield and long vase life of cut roses.