Women with chronic kidney disease (CKD) and chronic hypertension (CHT) frequently develop superimposed pre-eclampsia, but distinction from pre-existing disease is challenging. Plasma placental growth factor (PlGF), B-type natriuretic peptide (BNP), neutrophil gelatinase-associated lipocalin (NGAL), and serum relaxin concentrations were quantified in a longitudinal prospective cohort of 121 women with CKD: 44 with chronic hypertension, and 79 healthy controls. Biomarker concentrations were compared with 32 women with pre-eclampsia without pre-existing disease. Test performance was evaluated for diagnosis of superimposed pre-eclampsia requiring delivery within 14 days of sampling. PlGF was evaluated as a promising marker in a validation cohort of women with suspected pre-eclampsia (29 with CKD; 94 with chronic hypertension; 29 with superimposed pre-eclampsia requiring delivery within 14 days) and compared with women without pre-existing disease (290 with no pre-eclampsia and 176 with pre-eclampsia requiring delivery within 14 days). From 20 and up to 42 weeks of gestation, lower maternal PlGF concentrations had high diagnostic accuracy for superimposed pre-eclampsia requiring delivery within 14 days (receiver operator characteristic 0.85) and confirmed in the validation cohort. The other plasma and serum biomarkers were not discriminatory. Thus, plasma PlGF concentrations could potentially help guide clinical decision making regarding admission and delivery for superimposed pre-eclampsia.