The purpose of this study was to determine whether the application of a moist heat pack (MHP) could increase hamstring flexibility. Both legs for each subject were used for this study. Each leg was randomly assigned to either an MHP leg or a control leg condition. Twenty-seven male subjects (height = 178.5 +/- 8.6 cm; weight = 84.4 +/- 18.7 kg; age = 21.9 +/- 6.3 years) volunteered for this study. For the MHP leg condition, baseline hamstring flexibility was measured using an active knee extension test. A 23-ga indwelling thermistor was inserted to a depth of 2.54 cm to measure hamstring temperature. After baseline temperature was recorded, 2 MHPs were placed on either side of the thermistor until temperature was increased by 0.4 degrees C. Hamstring flexibility postintervention measurements were performed at 0, 4, 8, and 16 minutes. The same protocol was used for the control leg, without the MHP application. A 2 x 5 repeated-measures analysis of variance revealed no significant interaction between the MHP and the control leg condition. These results support previous findings that MHP application does not significantly affect muscle flexibility. After application of an MHP, it takes 20-25 minutes to increase intramuscular temperatures by 0.4 degrees C. Both of these findings should be taken into consideration when using an MHP to increase muscle flexibility.