We investigate the effect of marking-to-market on an optimal futures hedge under stochastic interest rates. An intertemporal optimal hedge ratio that accounts for basis risk and marking-to-market is derived. This ratio includes all previous hedge ratios, with constant interest rates as special cases. In a preliminary empirical study using S&P 500 index futures contracts, we demonstrate that the futures-forward hedging differential is nontrivial, especially in risk-return optimization. We also show that the covariances between interest rates and spot and futures prices explain the differential: the larger the covariances are, the larger the differential will be.