Preeclampsia is a disease associated with the maternal inflammatory response. YKL-40 and interleukin-18 (IL-18) are inflammatory markers involved in inflammatory states and vascular processes. The aim of this study was to evaluate serum levels of YKL-40 and IL-18 in preeclampsia. Twenty-four patients with preeclampsia and 13 healthy pregnant women were included in this study. Serum levels of YKL-40 and IL-18 were measured using an enzyme-linked immunosorbent assay (ELISA). Serum YKL-40 levels were significantly higher in patients with preeclampsia (median 75.0 ng/mL; range 45.8-125.4 ng/mL) than in normal pregnant women (median 29.3 ng/mL; range 16.2-39.5 ng/mL; p=0.000). Serum IL-18 levels were also significantly higher in patients with preeclampsia (median 159.9 pg/mL; range 125.6-193.5 pg/mL) than in normal pregnant women (median 120.2 pg/mL; range 102.2-157.9 pg/mL; p=0.018). Notably, higher serum YKL-40 levels were observed in patients with more severe proteinuria (>or=3+ by dipstick) than in patients with milder proteinuria (<or=1+ by dipstick; median 121.3 ng/mL, range 63.5-146.7 ng/mL versus median 55.4 ng/mL, range 29.4-116.3 ng/mL; p=0.022). There was, however, no significant difference in serum IL-18 levels between patients with more severe proteinuria and those with milder proteinuria. We conclude that circulating YKL-40 and IL-18 are elevated in patients with preeclampsia, and levels of YKL-40 are related to the severity of the disease.