Total adiponectin concentrations have been shown to increase in serum of preeclamptic women. However, variance of concentrations of different isoforms has not been studied, despite the emerging notion that high, medium and low molecular weight adiponectin exert different functions. We have determined serum concentrations of each adiponectin isoform using a newly developed enzyme immunosorbent assay. High molecular weight (HMW) adiponectin concentrations were significantly higher in women with preeclampsia (n=14; median, 11.2 microg/ml; interquartile range, 9.2-15.8) compared to normal pregnant women (n=14; 6.8 microg/ml, 5.4-10.7; P=0.04). In contrast, medium molecular weight and low molecular weight adiponectin concentrations were substantially equal between the groups. The ratio of HMW adiponectin to total adiponectin was also markedly higher in preeclamptic women (52.3%, 49.5-58.7) than control women (43.0%; 39.8-48.0; P=0.004). Taken together with other reports our findings imply a physiological feedback response to minimize endothelial damage in preeclamptic women.