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Erythrocyte cation fluxes were measured in fresh cells, on two different occasions, in 90 children and adolescents suffering from essential hypertension who were followed for a prolonged period. The purpose of the present study was to track the stability of erythrocyte cation fluxes with time, and to determine whether a known sodium transport abnormality can predict the severity of essential hypertension.The patients with an increased Na—Li countertransport were the most severely hypertensive in three different ways. Clinically, they presented a stable rather than a labile form of hypertension. Hemodynamically, the mean arterial pressure was higher than that of the other subgroups. Finally, in those children followed for more than 2 years, this subgroup remained hypertensive with time. The patients with a decreased Na,K cotransport activity were second in severity; however, blood pressure was often labile, mean arterial pressure was slightly lower than that of the previous subgroup and all patients remained hypertensive with time but in four out of six patients the hypertension was labile. The patients with increased passive sodium permeability presented a mild form of hypertension. The patients with normal erythrocyte cation fluxes seemed to form a heterogeneous group. A few were severely hypertensive, but in most the hypertension was borderline and transient. We conclude that determination of erythrocyte cation fluxes may be reproducible, and may be a useful index of the severity of essential hypertension in children and adolescents.