Half-hour returns are predictive of same half-hour returns in subsequent days. Using a unique dataset that provides masked trader identification and trader type, we find that the half-hour net order submission activity of institutional traders is positively and significantly associated with same half-hour returns on subsequent days, and that this relationship subsumes the return predictability in shorter intervals. We hypothesize that institutional net order activity is predictive of returns either because of repetitive order submission activity across days, or because of the slow diffusion of information that is initially revealed by institutional traders. Our evidence supports the former hypothesis.