SUMMARY
The white blood cell count and the erythrocyte sedimentation rate were studied in 180 hospital patients with a clear‐cut diagnosis of pertussis. In children below 6 months, the white blood cell picture favoured a diagnosis of pertussis in only one out of every four or five cases during the first 3 weeks of the disease, but not later. In children over 6 months, absolute leucocytosis and lymphocytosis were noted in the majority during the first 2 weeks of the disease (leucocytosis in 63 % and lymphocytosis in 71 %). During the third and fourth weeks changes in the white cells supported the diagnosis of pertussis in 80% and 50%, respectively. Marked leucocytosis (more than 50,000 leucocytes per mm3. of blood) was noted in 8 cases. I n uncomplicated pertussis the erythrocyte sedimentation rate is usually normal or subnormal and does not appear to vary with the patient's age or the week of the disease.