The Derifield-Thompson test for leg length inequality (LLI) is commonly used by chiropractors to assess a need for adjustment and to evaluate the results of adjustment. The two previous studies testing the reliability of the technique reported conflicting results. This study had two objectives: to demonstrate inter- and intraobserver reliability in detecting a LLI as little as 3 mm; and to document what effect Pierce-Stillwagon cervical adjusting has on a functional LLI. Twenty-six subjects walked into five successive examining rooms where a Derifield leg check was performed, including an estimate of the millimeters of difference in leg lengths. The subjects then entered a treatment room where they were randomly given no treatment, cervical adjusting, or gluteal massage. This process continued for 5 cycles. This study demonstrated that clinicians could reliably measure a LLI to less than 3 mm (both inter- and intraobserver), and also detect a change in LLI when the head was rotated. Neither cervical adjustment nor gluteal massage produced a significant change in observed LLI.