In order to assess the prognosis of hypertension first documented during labor, an inception cohort of all women with hypertension complicating pregnancy was assembled. In this cohort, even extreme elevations of blood pressure, regardless of the degree of proteinuria, were not associated with either maternal or fetal adverse outcomes. Therefore, some degree of hypertension during labor may be physiologic. Matched pairs analysis demonstrated that intravenous (IV) magnesium sulfate, administered to prevent seizures, was associated with an excess number of primary cesarean sections. Given the excellent prognoses in patients with good antepartum care in whom hypertension is first documented during labor, the need for any preventive measures, or for antihypertensive or anticonvulsant treatment, should be reconsidered.