Objective: To conduct a systematic review of evidence relating to management of mild chronic hypertension during pregnancy, including associated risks, benefits, and harms of treatment with antihypertensive agents, nonpharmacologic measures, and aspirin and benefits of various monitoring strategies. Data Sources: Using four broad search strategies, we searched English and non–English-language citations in 16 electronic databases from their inception to February 1999 and consulted relevant textbooks, references, and experts. Study Selection: Reviewers screened 6228 abstracts and found 215 articles that met multiple prespecified patient selection, study population, and design criteria. Tabulation, Integration, and Results: Forty-six studies consistently showed that chronic hypertension triples the risk for perinatal mortality (odds ratio [OR] 3.4; 95% confidence interval [CI] 3.0, 3.7) and doubles the risk for placental abruption (OR 2.1; 95% CI 1.1, 3.9). Thirteen small, randomized controlled trials had inadequate power to rule in or rule out moderate-to-large (20%–50%) benefits of antihypertensive treatment. Possible adverse effects were fetal renal failure when angiotensin-converting enzyme inhibitors are used in the second or third trimester and growth restriction when atenolol is used early in pregnancy. Trials showed that aspirin neither reduces nor increases perinatal and maternal morbidity, but they did not rule out possible small-to moderate beneficial or adverse effects. No studies provide guidance on benefits or consequences of various nonpharmacologic therapies or monitoring strategies. Conclusion: Mild chronic hypertension is associated with increased maternal and fetal risks. Beneficial treatment and monitoring regimens are not clear, but some treatments, such as angiotensin-converting enzyme inhibitors, are best avoided.