The total, lipid and protein-bound sialic acid in serum was estimated in 32 healthy pregnant controls and in 32 patients with hydatidiform moles. Data analysis indicated a significant increase in the total and protein-bound sialic acid levels in patients with hydatidiform moles (p < 0.001) when compared to healthy pregnant controls in the same trimester of pregnancy. No significant difference was observed between controls and patients for lipid bound-bound sialic acid levels. When total, protein and lipid-bound sialic acid values were normalized to protein, significant increases were also observed in patients with hydatidiform moles except for lipid-bound sialic acid. The findings described here strongly suggest that serum total and/or protein-bound sialic acid/total protein ratio is a better indicator of the presence of trophoblastic disease than is lipid-bound sialic acid levels in patients with hydatidiform mole.