  A modular form for an even lattice L of signature (2,n) is said to be
2-reflective if its zero divisor is set-theoretically contained in the Heegner
divisor defined by the (-2)-vectors in L. We prove that there are only finitely
many even lattices with n>6 which admit 2-reflective modular forms. In
particular, there is no such lattice in n>25 except the even unimodular lattice
of signature (2,26). This proves a conjecture of Gritsenko and Nikulin in the
range n>6.
