By thermal decomposition in the presence only of oleylamine, sub-10 nm hexagonal NaLuF4-based nanocrystals codoped with Gd3+, Yb3+, and Er3+ (or Tm3+) have been successfully synthesized. Sub-10 nm beta-NaLuF4: 24 mol % Gd3+, 20 mol % Yb3+, 1 mol % Tm3+ nanocrystals display bright upconversion luminescence (UCL) with a quantum yield of 0.47 +/- 0.06% under continuous-wave excitation at 980 nm. Furthermore, through the use of beta-NaLuF4: Gd3+,Yb3+,Tm3+ nanocrystals as a luminescent label, the detection limit of <50 nanocrystal-labeled cells was achieved for whole-body photoluminescent imaging of a small animal (mouse), and high-contrast UCL imaging of a whole-body black mouse with a penetration depth of similar to 2 cm was achieved.