The role of temperature in bulb dormancy induction and release was investigated in one-year-old seedlings of three populations of Lilium longiflorum, i.e., Yakushima (LYA) and Kikaijima (LKI) in the Ryukyu Archipelago, Japan and Pitouchiao (LPI), Taiwan, and two populations of L. formosanum, Wulai (FWU), Taiwan and domesticated Fukuoka population (FFU), Japan. LYA and LKI undergo deep summer dormancy, while LPI, FWU and FFU do not. Four weeks of high temperature induced dormancy in LYA and LKI, whereas two weeks of high temperature was insufficient. Dormancy was not induced in LPI and FFU even after eight weeks of high temperature. Re-growth after dormancy was initiated in LKI and LYA six to eight weeks after shifting to 15°C. In the temperature-controlled experiment, all the populations continued developing new leaves at 15°C for 22 weeks, and stopped leaf production at 20 and 25°C in winter, while FFU produced new leaves at 30°C. It is considered that summer dormancy induction and release in one-year-old seedlings of northern ecotypes of L. longiflorum is determined by the duration of high and low temperature, respectively. It was proved that development of non- or weakly dormant ecotypes of L. longiflorum and L. formosanum is associated with the lack or reduction of sensitiveness to high temperature.