Our objectives were to examine interrater reliability for inclinometer and goniometric measurements of hip extension flexibility and the intra- and interrater reliability between instruments when measuring hip extension flexibility with the modified Thomas test. The modified Thomas test is frequently used by clinicians to assess hip extension flexibility. Studies examining the reliability of measures of the modified Thomas test are limited. This study included 42 healthy subjects. The modified Thomas test was performed on each subject to quantify hip extension flexibility by using an inclinometer and a goniometer. High interrater reliability was determined for each instrument (r = 0.91-0.93; ICC = 0.89-0.92). High intrarater parallel-forms reliability was found for each examiner between the instruments (r = 0.089-0.92; ICC = 0.91-0.93). High interrater parallel-forms reliability also was found between instruments (r = 0.86-0.93; ICC = 0.86-0.92). The high correlations obtained in this study suggest that the inclinometer and the goniometer are reliable instruments for measuring hip extension flexibility. The results also indicate that the two instruments can be used interchangeably for measuring hip extension flexibility.