Recently it has been suggested that TGF- g 1 may play an important role in trophoblast invasion of spiral arteries in pre-eclampsia. This study was designed to investigate TGF- g 1 levels in pre-eclampsia. Platelet depleted plasma TGF- g 1 levels were measured by ELISA in 43 normotensive healthy non-pregnant women, 30 normotensive healthy pregnant and 42 pre-eclamptic women: TGF- g 1 levels were increased significantly in the pre-eclamptics (4·9 - 0·9 ng/ml) compared to the normotensive pregnant (1·9 - 0·54 ng/ml; P <0·001) and normotensive non-pregnant groups (2·1 - 0·41 ng/ml; P <0·0001). There was no significant difference in TGF- g 1 levels between normotensive pregnant and non-pregnant groups (1·9 - 0·54 vs. 2·1 - 0·41 ng/ml; P =0·2). There was a highly significant correlation between neonatal weight ( r =0·63; P <0·0001) and TGF- g 1 levels. The finding of increased levels of plasma TGF- g 1 suggests an aetiological role of this peptide in pre-eclampsia.