Objective: The aim of the present study is to investigate whether alterations in the serum levels of apelin and YKL-40 differ between early and late onset pre-eclampsia and whether there is a correlation between apelin and YKL-40 in women who subsequently develop early and late pre-eclampsia. Materials and methods: A total number of 80 pregnant women, 40 with normal pregnancy and 40 with pre-eclampsia, were included in the present study. Both the normal pregnant and pre-eclamptic subjects were subdivided into two groups. Serum YKL-40 and apelin concentrations were measured. Results: Mean maternal serum YKL-40 levels were both lower in women who subsequently developed early (87.45 +/- 3.07 versus 103.40 +/- 4.29) or late (96.43 +/- 4.06 versus 99.87 +/- 3.63) pre-eclampsia than those who remained normotensive. The difference was significant in early-onset preeclamptic women (p<0.05) rather than late-onset pre-eclamptic ones (p>0.05). Mean maternal serum apelin levels were both higher in women who subsequently developed early (8.6 +/- 3.6 versus 5.7 +/- 1.2) or late (9.6 +/- 2.5 versus 8.1 +/- 1.8) pre-eclampsia than those who remained normotensive. The difference was significant in early-onset preeclamptic women (p<0.05) rather than late-onset pre-eclamptic ones (p>0.05). There was a significant negative correlation between serum apelin and YKL-40 levels (r=-0.48, p=0.001). Conclusion: Circulating levels of apelin are significantly increased in early-onset pre-eclampsia, indicating the role of apelin in the discrimination of the early-onset of pre-eclampsia. On the other hand, maternal serum YKL-40 levels are not elavated significantly, indicating that adipose-derived apelin is primarily involved in the vascular pathogenesis of early-onset pre-eclampsia than macrophage-derived YKL-40.