Calorimetric analysis depends on the direct proportionality between the heat changes that occur during chemical reactions and the amount of reacting substances. Potential uses of calorimetry in the clinical laboratory are discussed, with examples. The calorimetric technique does not require optically clear specimens, and if the specificity of the measured reaction is assured, calorimetry can be used for quantitative determinations of components that are present in a complex matrix system such as body fluids. Specific enzymic reactions have been used to measure substrates and enzyme activities in biological specimens calorimetrically, with precision, sensitivity, and accuracy comparable to routine photometric techniques. The application of calorimetry in the clinical laboratory is limited now by its slowness, but development of automated instruments may enable the technique to become competitive with conventional analytical techniques in the clinical laboratory.