An erythrocyte sedimentation rate (ESR) value is principally dependent on the concentration of plasma fibrinogen. The ESR indirectly reflects the concentration of the acute phase proteins in the plasma. Its value may significantly be affected by the number, size and morphology of erythrocytes as well as the amount of other plasma components, for example immunoglobulins. The significance of a slightly elevated ESR value is difficult to assess, particularly in the elderly. The threshold for starting investigations should be set considerably higher if the patient remains asymptomatic or if the ESR has not clearly increased from previous readings. The level of the ESR is dependent on the activity of the disease, and it is often a useful investigation when monitoring disease progress. In many situations the CRP concentration decreases along with the disease activity, but the ESR remains high, sometimes permanently, because of hypergammaglobulinaemia.