['The objective of this case-cohort study was to evaluate the relationship between maternal 25-hydroxyvitamin D (25(OH)D) concentration and preeclampsia overall and by severity.', 'From an eligible cohort of 12,861 women who had serum banked from aneuploidy screening in Pittsburgh, Pennsylvania from 1999 to 2010, we randomly sampled a subcohort of 2327 pregnancies and all remaining preeclampsia cases (n\xa0= 650 cases). Preeclampsia (defined as new-onset hypertension and proteinuria) and its mild and severe forms were identified using ICD-9 codes. Maternal serum collected at 20\xa0weeks or less gestation was measured for 25(OH)D. We used log-binomial regression with restricted cubic splines to estimate the association between 25(OH)D and preeclampsia after adjusting for confounders.', 'Approximately 21% of the randomly selected sample had 25(OH)D less than 50\xa0nmol per L. We found that the adjusted risk of preeclampsia declined as serum 25(OH)D increased to 50\xa0nmol per L and then plateaued (test of nonlinearity P\xa0<\xa0.05). The adjusted preeclampsia risk ratios (95% confidence intervals) for 25(OH)D less than 25\xa0nmol per L, 25 to 49.9\xa0nmol per L, and 50 to 74.9\xa0nmol per L were 2.4 (1.2-4.8), 1.1 (0.69-1.7), and 1.3 (0.89-1.8), respectively, compared with those with 25(OH)D 75\xa0nmol per L and over. Similar associations were observed with severe and mild preeclampsia.'] Does low maternal 25-hydroxyvitamin D concentration increase the risk of severe and mild preeclampsia?