With the ligands 4-phenyl-1-(furan-2-carbaldehyde)thiosemicarbazone, HTSC1, (1), 4-phenyl-1-(5'-phenyl-furan-2-carbaldehyde)thiosemicarbazone, HTSC2 (2), o-methoxy-benzaldehydethiosemicarbazone, HTSC3 (3), and o-cyano-benzaldehydethiosemicarbazone, HTSC4 (4), the corresponding palladium(II) complexes, Pd(TSC1)(2) (5), Pd(TSC2)(2) (6), Pd(TSC3)(2) (7), and Pd(TSC4)(2) (8) were synthesized and characterized by elemental analysis and spectroscopic techniques. The crystal structure of Pd(TSC3)(2) (7) was determined by single-crystal X-ray diffraction. Complex 7 shows a square-planar geometry, where two deprotonated ligands are coordinated to the Pd-II center through the nitrogen and sulfur atoms in a trans arrangement. In vitro antitumor studies against different human tumor cell lines have revealed that the pallaclium(II) complexes 5-8 are more cytotoxic (IC50 values in the range of 0.21-3.79 mu m) than their corresponding ligands (1-4) (> 60 mu m). These results indicate that the antiproliferative activity is enhanced when thiosemicarbazone ligands are coordinated to the metal. Among the studied palladium(II) complexes, 8 exhibits high antitumor activity on K562 chronic myelogenous leukemia cells with a low value of the inhibitory concentration (IC50 = 0.21 mu m).