HL-60 human promyelocytic leukemia cells were exposed to 1, 3, and 10 MHz ultrasound signals (alone or in combination) for 0 to 20 minutes under different temperature conditions between 30 - 36°C and evaluated for gene expression, either 4 or 24 hours after treatment. The authors report increased Hsp72, Hsp32, and Hsp40 expression with 10 MHz exposure for at least 5 minutes at the higher temperatures. Pulsed exposure patterns (1:2, 5 ms on and 5 ms off) induced Hsp72 to the same level, whereas other pulsed schemes (1:5, 2 ms on and 8 ms off and 1:10; 1 ms on and 9 ms off) had no effect. Sonication of cells for 20 minutes increased Hsp72 expression if it was performed continuously or in 2 x 10 minute periods, but reduced Hsp72 expression when performed for 4 x 5 minute periods. The authors conclude the effects of ultrasound exposure are enhanced by increased culture temperature. In related studies, the authors report on cell survival with ultrasound exposure.