Summary.In healthy subjects the ESR is higher in women than in men, and in both sexes a rise with age occurs. It is well established that pathological elevation of the ESR may be due to elevation of the fibrinogen level. The present study has shown that in normal subjects the ESR correlates not only inversely with the haemoglobin level but also directly with the fibrinogen level. These two factors together explain the difference in ESR between men and women, and the major part of the age‐related increase in the ESR. Fibrinogen levels were found to be significantly higher in male smokers than male non‐smokers, and in women were inversely correlated with alcohol consumption; despite this the ESR did not differ in smokers and non‐smokers, nor was it related to alcohol intake. Oral contraceptive intake influenced neither the fibrinogen level nor the ESR. The correlation of ESR and fibrinogen concentration in healthy subjects suggests that physiological variation in the ESR is determined by the fibrinogen concentration as well as the haemoglobin