Different thermal techniques have been used to increase hamstring flexibility with varying results. Research has shown that cold will help increase hamstring flexibility; however, there are limited studies on the use of cold treatments to influence hamstring flexibility. This study tested whether cold packs will impact hamstring flexibility. Subjects were healthy male or female 18-30 year olds. Subjects were excluded based on answers to a medical questionnaire, Body Mass Index>30, and hamstring injury within a year. Of thirty potential subjects, 29 fit the requirements of the study. Subjects marched in place to insure a common pre-test condition. A goniometer was used to measure hamstring flexibility. A passive straight leg raise was done to maximum tension and subject discomfort. Three pre and three post cooling measurements were taken bilaterally. Only one leg was cooled with a 2:1 water to alcohol cold pack. A repeated measures ANOVA and post hoc comparisons with paired t-tests were used to analyze the data. Significant (p<.05) differences between the pre and post cooled hamstring measurements were found. No significant differences occurred between pre and post test measurements on the uncooled leg. Observations from this study showed flexibility can be increased by applying a cold pack to the hamstrings. This research based evidence may be used in the practice of physical therapy when considering a plan of care that includes increasing hamstrings flexibility in 18-30 year olds.