Six-meridians syndrome differentiation, a method developed by Zhang Zhongjing, a celebrated doctor in the Han Dynasty, is the principle for syndrome differentiation and treatment in Treatise on Seasonal Febrile Disease and is the basis of syndrome differentiation for the later generations.

Six-meridians syndrome differentiation categorizes the stages of exogenous febrile diseases into six types for selection of treatment according to the main principle of yin and yang, namely taiyang disease, yangming disease, shaoyang disease, taiyin disease, shaoyin disease and jueyin disease.

Six-meridians diseases reflect the pathological changes of the meridians and viscera. Among the six types of diseases, taiyang disease pertains to the external, yangming disease to the internal, shaoyang disease to the semi-external and semi-internal; while the three yin types all pertain to the internal. The three yang types of disease reflect the pathological changes of the six fu organs, while the three yin types of diseases reflect the pathological changes of the five zang organs. So the six-meridians diseases include the pathological changes of both the twelve meridians and viscera. Since the six-meridians syndrome differentiation focuses on the analysis of the pathological changes and transmission rule of diseases caused by exogenous wind-cold, they are not identical with syndrome differentiation of viscera.