A raised erythrocyte sedimentation rate (ESR) is considered a hallmark for the diagnosis of giant cell arteritis (GCA). The American College of Rheumatology 1990 criteria for GCA include ESR greater than or equal to 50 mm/h as one of the five criteria. Although the presence of a normal ESR made GCA less likely, the results of a population-based study showed that the occurrence of a low/normal value in GCA at diagnosis is not rare. Pre-treatment ESR may be a prognostic indicator for duration of treatment. C-reactive protein (CRP) and interleukin-6 (IL-6) may be more sensitive indicators of disease activity than ESR in GCA patients. However, it is unclear whether the use in clinical practice of CRP and IL-6 has some apparent advantage over ESR. ESR is the most often used tool to assess disease activity in Takayasu's arteritis (TA). However, some studies have found that ESR and CRP are not able to differentiate patients with clinically active and inactive TA. Furthermore, histopathological studies have shown that over 40% of patients thought to be in clinical remission with normal acute phase reactants have active arteritis. IL-6 could be a promising marker of disease activity in TA; however, further studies are required to confirm its usefulness in clinical practice. Other laboratory investigations could be useful in the diagnosis or follow-up of GCA and TA, but more studies are required.