Deterministic chaos is a pattern of fluctuations that may seem to be stochastic (caused by random external forces) but is actually produced, in a determined manner, by the action of nonlinear dynamic processes. The existence of this phenomenon has important implications for pathologists because it means that experimental data with apparently random fluctuations may be caused by a deterministic process. The patterns noted in deterministic chaos are very sensitive to variation in initial conditions, so if chaotic processes do occur in pathology then methods of predicting biological behavior (prognosis) of lesions, such as tumors, will have to be modified. This review shows the properties of deterministic chaos using a simple mathematical model of tumor growth and describes the methods that may be used to analyze experimental data for the presence of such chaos illustrating these with some clinical data. Established applications in medicine are reviewed and antichaotic mechanisms are discussed.