This paper describes the fabrication and evaluation of folic acid (FA)-conjugated nanodiamond (ND) nanoclusters for selective photothermal tumor therapy. ND nanoclusters with surface carboxyl groups are aminated using ethylenediamine and conjugated with FA via carbodiimide chemistry. The temperature of an aqueous ND dispersion (10 mu g mL(-1)) is increased to 54 degrees C upon laser exposure for 5 min. FA-ND nanoclusters are preferentially taken up by KB cells (folate receptor positive) compared to WI-38 (folate receptor negative) cells, suggesting specificity for tumor cells that overexpress folate receptors. Cell viability tests reveal that FA-ND nanoclusters effectively and selectively ablate KB cells upon near-infrared (NIR) laser exposure. In addition, fluorescence microscopy images confirm that only KB cells treated with FA-ND nanoclusters are ablated in a spot (200 mu m in diameter) by NIR laser exposure. In an animal model, a large amount of FA-ND nanoclusters is accumulated into tumor tissue, resulting in dramatically reduced tumor volume post-NIR laser exposure as compared to ND nanoclusters.