Hypocotyl growth occurs as a result of an interaction between environmental factors and endogenous phytohormones. In Arabidopsis, high temperature promotes auxin synthesis to increase hypocotyl growth. We previously showed that exogenously provided auxin stimulates expression of the brassinosteroid (BR) biosynthetic gene DWARF4. To determine whether temperature-induced hypocotyl elongation depends on BR biosynthesis, we examined the morphological responses to high temperature and the expression pattern of DWF4pro:GUs in different genetic backgrounds, which are as follows: Ws-2 wild-type, iaa19/msg2, bri1-5, and Dwf7-1. In contrast to the wildtype, growth of the three genotypes at $29^{circ}C$ did not significantly increase hypocotyl length; whereas, with the exception of iaa19/msg2, the roots were elongated. These results confirm that BR biosynthesis and signaling pathways are required for hypocotyl growth at high temperature. Furthermore, a GUS histochemical assay revealed that a temperature of $29^{circ}C$ greatly increased DWF4pro:GUs expression in the shoot and root tips compared to a temperature of $22^{circ}C$. Quantitative measurements of GUS activity in DWF4pro:GUs revealed that growth at $29^{circ}C$ is similar to the level of growth after addition of 100 nM IAA to the medium. Our results suggest that temperaturedependent synthesis of free auxin stimulates BR biosynthesis, particularly via the key biosynthetic gene DWF4, and that the BRs thus synthesized are involved in hypocotyl growth at high temperature.