The goal of this study is to utilize near-infrared spectroscopy as a non-invasive, inexpensive method of diagnosing deep vein thrombosis. The probe contains two light sources and two filter detectors that record monitor deoxyhemoglobin and oxyhemoglobin changes via the monitoring of reflected light of wavelengths 760 and 850 nm. These changes and the sum of these changes, which corresponds to blood volume changes, are plotted over a time period in which the subject undergoes a series of light exercises. The test protocol is designed to determine the muscle tissue's blood volume capacity, rate of filling and efficiency to promote one-directional venous flow. The subject pool consists of Johns Hopkins Hospital Vascular Surgery patients diagnosed with leg deep vein thrombosis with normal subjects as the control. Abnormal venous systems have distinct characteristics: high blood volume; quick rate of filling; and, the inability to decrease the blood volume during the contraction of the muscle.