Depends on how you count. The endowment is mainly (all?) Gates's money, but a substantial proportion of the annual spending is funded by Buffett. That's because, rather than donate money to the Foundation outright, Buffett set up a recurring-donation program structured in a way so it feeds directly into the Foundation's annual program spending.

For 2011, it looks like the Foundation spent $3.4 billion total, of which the annual installment of Buffett's donation accounted for $1.5 billion, so a little under half.