Hippocrates (460-370 BC) propounded the concept of humours to define the state of health and disease of human body, and classified them in four groups based on their colours. This theory was further disseminated by Galen and Avicenna. Humours in Unani practice are named as dam (blood/ sanguine), balgham (phlegm), safra (yellow bile), and sawda (black bile). Each person is considered to have a unique humoral makeup, an admixture of all four humours with an ascendance of one of them which actually acts as the representative of all humours in the body and the person is said to have damwi, balghami, safrawi or Sawdawi temperament. Among the four humours, dam is considered the most worthy and superior. Tabi‘i dam (Normal blood) is one in which all humours are in desirable proportion with respect to quantity and quality. It is normal in colour, moderate in viscosity and taste, free from bad odour, sepsis and pepsis etc. Tabi‘i dam equipoise in quantum and quality guarantees good health. It has been mentioned that mizäj (temperament) of organs remain normal only when the nourishing blood is normal. Ancient Unani physicians have described the causes, pathological changes, signs, symptoms and treatment of abnormal blood. In the present paper an attempt has been made to develop a comprehensive understanding of ghayr tabi‘i dam (morbid blood) and its associated disorders in the light of the descriptions mentioned in Unani literature.