While the presence and distribution of galectin-1 and galectin-3 in different normal trophoblast cell populations is known, no information is available regarding their occurrence in malignant trophoblast of gestational trophoblastic disease (GTD). Galectins-1 and -3 have, however, been implicated in malignancies of other tissues. Immunoreactivity for these galectins in the transformed trophoblast of invasive mole (n = 8), choriocarcinoma (n = 7) and one case of placental site trophoblastic tumor (PSTT) was compared to that of the invasive trophoblast of the normal first trimester of pregnancy implantation sites (n = 9). A large proportion of the transformed trophoblast cells of all GTD studied were positive for galectin-1 and galectin-3. Immunoreactivity was scored semiquantitatively to include both the prevalence among the trophoblast cells and the intensity of staining. Immunoreactivity for both galectin-1 and galectin-3 in gestational trophoblastic disease is increased (significant differences at p < 0.05, Mann-Whitney Rank Sum Test). This finding may suggest a possible implication of galectins-1 and -3 in the invasiveness of the transformed trophoblastic cell, although the exact physiological significance of this finding remains to be determined.