The use of thermography in evaluating nerve injury is based on the presence of temperature asymmetries between the involved area of innervation and the corresponding area on the opposite side of the body. However, interpretation of the thermographic image has been troubled by subjectivity. This paper describes a computer-calculated method of collecting data that eliminates subjective biases. Comprehensive normative data are presented on the degree of thermal asymmetry in the human body. The degree of thermal asymmetry between opposite sides of the body (delta T) is very small. For example, the value of delta T for the forehead (mean +/- standard deviation) was 0.18 degree +/- 0.18 degree C, for the leg it was 0.27 degree +/- 0.2 degree C, and for the foot it was 0.38 degree +/- 0.31 degree C. These values were reproducible in both short- and long-term follow-up measurements over a period of 5 years. The delta T's reported here were obtained from 40 matched regions of the body surface of 90 asymptomatic normal individuals. These values can be used as a standard in assessment of sympathetic nerve function, and the degree of asymmetry is a quantifiable indicator of dysfunction.