The haemodynamic changes in essential hypertension were investigated in 7 girls and 4 boys under the age of 18 years. The Fick method was used for the estimation of cardiac output (CO). The cardiac indices (CIs) ranged form 2.8 to 6.6 litres/min/m<sup>2</sup> and the total peripheral resistance (TPR) from 1,100 to 2,860 dyn/s/cm<sup>-5</sup>·m<sup>2</sup>. Comparison of the results with normal values reported earlier showed that the CIs in the present series were increased in 2 patients and normal in the others. The stroke volume was increased in 3 patients. TPR was elevated in 4 patients. It is proposed that essential hypertension does not always begin with an increase in CO, and that some forms of essential hypertensions can have a high TPR already at the onset of the disease.