Platelet distribution width (PDW) is a component of a normal FBC. It reflects a variation in the size distribution (platelet anisocytosis). Normal range of PDW is 9.6 to 15.3 femtolitres. PDW tend to be increased in inflammatory reaction and clinical diseases. Platelet activation increases PDW. Inflammatory cytokines [interleukin-6 (IL6), granulocytes colony stimulating factor (G-CSF) and macrophage colony stimulating factor (M-CSF)] regulate megakaryocytic maturation, platelet production and platelet size. It can be used for the diagnosis of immune thrombocytopenia. Increased PDW values have been reported in patients with a wide range of diseases; Diabetes mellitus; Cancer; Cardio-cerebrovascular and respiratory disorders. Increased morbidity and mortality among patients with critical illness, coronary artery disease, cancer, pulmonary embolism and chronic obstructive pulmonary disease.