  We study the elliptic genera of hyperKahler manifolds using the
representation theory of N=4 superconformal algebra. We consider the
decomposition of the elliptic genera in terms of N=4 irreducible characters,
and derive the rate of increase of the multiplicities of half-BPS
representations making use of Rademacher expansion. Exponential increase of the
multiplicity suggests that we can associate the notion of an entropy to the
geometry of hyperKahler manifolds. In the case of symmetric products of K3
surfaces our entropy agrees with the black hole entropy of D5-D1 system.
