closo-Dodecaborate-encapsulating liposomes, which are high boron content liposomes, were developed as a boron delivery vehicle for neutron capture therapy. The use of spermidinium cation (spd) as an alternative counter cation of closo-dodecaborates was essential for the preparation of high boron content liposome solutions; final boron concentration in the liposome solutions was approximately 14,000 ppm and B/P ratio was 3.5. The spd cation prevented the interaction of anionic closo-dodecaborates ([B12H12]2-) with the phosphatidylcholine moiety of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) in the liposomes, thereby suppressing the release of liposome contents and increasing liposome yield. Tumor boron concentrations reached 203 and 242 ppm 36 h after injecting colon 26 tumor bearing mice with 100 mg B/kg of spd-[B12H11SH] (spd-BSH) and spd-[B12H11NH3] encapsulating liposomes, respectively. Tumor boron concentrations of 34.0 and 35.4 ppm were noted 36 h after injecting 15 mg [B]/kg of spd-BSH- and spd-[B12H11NH3]-encapsulating liposomes, respectively. The tumor growth of the mice that received a single injection even at the dose of 15 mg [B]/kg was significantly suppressed and locally controlled for 100 days after thermal neutron irradiation. The total amount of liposomes was reduced to less than one-seventh of the amount of liposomes used to prepare Na2BSH-encapsulating liposomes.