Microscopic surface tensions of cavities in liquids or of droplets with radii down to about 0.5 nm have been predicted. Surface energies of small clusters are calculated from the unretarded dispersion forces at a spherical interface. The resulting expression for surface tension is a function of basic intermolecular properties and cluster size, with predictions for macroscopic surface tensions being in good agreement with experimental data (flat surface). The surface tension is found to decrease with decreasing radius, and for n-alkanes, such size effects occur below 2 nm; above this size, the predicted results closely approximate the planar values.