This study evaluated the relationship between the activity of the inflammatory indicator adenosine deaminase (ADA) in placental tissue and maternal and fetal (umbilical cord) plasma and the severity of pre-eclampsia. Maternal and umbilical vein whole blood and placental tissue samples were collected from women with normal pregnancies (n = 33) and patients with mild (n = 12) or severe (n = 17) pre-eclampsia. ADA activity was measured spectrophotometrically. Significantly increased ADA activity was detected in maternal and fetal plasma, and placental tissue in patients with mild and severe pre-eclampsia compared with normal pregnancies; there were no significant differences between the mild and severe cases. The presence of increased ADA activity in pre-eclampsia is consistent with activation of the inflammatory system in this condition. The increased ADA activity was related to the presence of the disease but not the severity of clinical symptoms. Neonatal outcome did not significantly correlate with observed ADA activity.