The erythrocyte sedimentation rate (ESR) remains a commonly measured indicator of disease, but is subject to several non-disease influences. The haemoglobin concentration (Hb) and ESR were measured in 1249 consecutive patients (492 men, 757 women) from primary care practices. An inverse correlation was found between Hb and ESR throughout the range of measured Hb, and in particular there was a significant difference in the median ESR of patients in the highest and lowest quartile for non-anaemic Hb (P < 0.001). These results indicate that correct clinical analysis of an ESR result should take into account the Hb, both in anaemic and in non-anaemic patients. Interpretative difficulties due to external influences on the measured ESR could be resolved by replacement of this test with plasma viscosity estimation.