The effects of the formulation and particle composition of gadolinium (Gd)-containing lipid nanoemulsion (Gd-nanoLE) on the biodistribution of Gd after its intravenous (IV) injection in D1-179 melanoma-bearing hamsters were evaluated for its application in cancer neutron-capture therapy. Gd-nanoLEs whose particles had an oily core (soybean oil, ethyl oleate, lipiodol, or triolein) and a surface layer of hydrogenated phosphatidylcholine, gadolinium-diethylenetriaminepentaacetic acid-distearylamide, and a cosurfactant (Myrj 53, Brij 700, or HCO-60) were prepared by a thinlayer hydration-sonication method. Biodistribution data revealed that Brij 700 and HCO-60 prolonged the retention of Gd in the blood and enhanced its accumulation in tumors. Among the core components employed, soybean oil yielded the highest Gd concentration in the blood and tumor and the lowest in the liver and spleen. Gd-nanoLEs with a Gd content of 1.5-4.5 mg/ml could be formulated by using HCO-60 and soybean oil at a constant oil-to-water ratio, and by enriching Gd in the surface layer with the particle size maintained below 100 nm. When each Gd-nanoLE was IV injected once or twice at a 24-h interval, the Gd concentration in the tumor correlated well with the total dose of Gd, and it reached a maximum of 189 μg/g wet tumor. This maximum Gd level was greater than the limit required for significantly suppressing tumor growth in neutron-capture therapy.