  It is known that every oriented integral homology 3-sphere can be obtained
from S^3 by a finite sequence of Borromean surgeries.
  We give an explicit formula for the variation of the Casson invariant under
such a surgery move. The formula involves simple classical invariants, namely
the framing, linking number and Milnor's triple linking number.
  A more general statement, for n independent Borromean surgeries, is also
provided, which involves an additional cubic expression in some linking numbers
of the surgery link.
