OBJECTIVE: To examine the relationship between baseline renal function tests in pregnant patients with chronic hypertension and adverse pregnancy outcomes. METHODS: This was a retrospective cohort study of women with a singleton pregnancy and chronic hypertension with assessment of renal function (urine protein-to-creatinine ratio and serum creatinine) before 20 weeks of gestation. The primary outcome was severe preeclampsia at less than 34 weeks of gestation. Secondary outcomes were severe preeclampsia at any gestational age, any preeclampsia, preterm birth at less than 35 weeks of gestation, composite perinatal outcome (perinatal death, assisted ventilation, cord pH less than 7, 5-minute Apgar score 3 or less, or neonatal seizures), and small for gestational age. The association between baseline renal function and the primary outcome was assessed with receiver operating characteristic (ROC) curves and objective cutoffs determined. Outcomes were compared between those with values above and below the cutoffs using univariable and multivariable analyses. RESULTS: Seven hundred fifty-five pregnant women with chronic hypertension had baseline renal function assessment. The urine protein-to-creatinine ratio and creatinine cutoffs for severe preeclampsia at less than 34 weeks of gestation were 0.12 or greater and 0.75 mg/dL or greater, respectively. The area under the ROC curves for severe preeclampsia at less than 34 weeks of gestation was 0.74 (95% confidence interval [CI] 0.7-0.8) for urine protein-to-creatinine ratio and 0.67 (95% CI 0.6-0.8) for creatinine. A urine protein-to-creatinine ratio 0.12 or greater was associated with an increased risk of developing severe preeclampsia at less than 34 weeks of gestation (16.4% compared with 2.6%, adjusted odds ratio [OR] 7.5, 95% CI 3.9-14.6) as was a creatinine 0.75 mg/dL or greater (15.7% compared with 4.6%, adjusted OR 3.5, 95% CI 1.9-6.3). Severe preeclampsia at less than 34 weeks of gestation occurred in only 1.6% of patients if their baseline tests were below both cutoffs. CONCLUSION: Baseline assessment of renal function can be used as a risk stratification tool in pregnant patients with chronic hypertension.