The objective is to test the hypothesis that serum IL-12 concentrations would be elevated in women with severe pre-eclampsia and HELLP syndrome. The Methods used were as follows: Serum was obtained from women admitted to our Labor and Delivery unit diagnosed with severe pre-eclampsia or HELLP syndrome and normal control patients. IL-12 concentrations in these samples were determined by the use of two different and specific enzyme-linked immunosorbent assays for the p40 subunit and the intact p75 dimer. It was found that serum IL-12 (p40 subunit) concentrations were elevated in women with both severe pre-eclampsia (p = 0.011) or HELLP syndrome (= 0.004). Similar findings were noted for these patients when matched with control patients for maternal age, gestational age, and parity. Eleven women had elevations of serum IL-12 p75 dimer, and 10 of these 11 patients had severe pre-eclampsia or HELLP syndrome. In conclusion, we found that women with severe pre-eclampsia and HELLP syndrome commonly have detectable concentrations of the IL-12 p40 monomer and were more likely than normal control women to have detectable serum IL-12 p75 dimer. While the exact role of IL-12 in hypertensive disease during pregnancy is unclear, our data support the hypothesis that the regulation of IL-12 production and metabolism is abnormal in women with pre-eclampsia and HELLP syndrome, perhaps contributing to the immunologic alterations characteristic of these disorders.