OBJECTIVE: The aim of this study is to determine the relationship between heart rate and/or blood pressure variability, measured at 28 weeks' gestation, and the incidence of pregnancy-induced hypertension or preeclampsia. STUDY DESIGN: Secondary analysis of data from a prospectively enrolled cohort of 385 active military women in whom spectral analysis of continuous heart rate and variability was measured at 28 weeks' gestation. The primary outcome was the predictive value of spectral analysis of heart rate and blood pressure for hypertensive diseases of pregnancy. RESULTS: High-frequency heart rate variability was reduced and low-frequency variability of systolic and diastolic blood pressure increased in women who would develop pregnancy-induced hypertension but not preeclampsia. Low-frequency variability of diastolic blood pressure remained a significant predictor of pregnancy-induced hypertension but not preeclampsia after adjustment for age, weight, and blood pressure in a multivariate model. CONCLUSION: Early identification of pregnancy-induced hypertension can facilitate treatment to avoid maternal morbidity. Understanding the physiological underpinnings of the two very different diseases may lead to improved treatment and prevention. If proven effective in a broader population, the ability to differentiate pregnancy-induced hypertension from preeclampsia may reduce unnecessary iatrogenic interventions or inappropriate preterm delivery.