  Two Dehn surgeries on a knot are called {\it purely cosmetic}, if they yield
manifolds that are homeomorphic as oriented manifolds. Suppose there exist
purely cosmetic surgeries on a knot in $S^3$, we show that the two surgery
slopes must be the opposite of each other. One ingredient of our proof is a
Dehn surgery formula for correction terms in Heegaard Floer homology.
