BOSTON (Reuters) - Stephen Blyth, who had been appointed to oversee Harvard University's $37.6 billion endowment only 18 months ago, has resigned, the Ivy League school said on Wednesday.

Blyth, who took a temporary medical leave in May, is departing for personal reasons, the school said.

Robert Ettl, chief operating officer at Harvard Management Co, will continue to run the university's investment arm on an interim basis as the board looks for a permanent successor for Blyth.

(Reporting by Svea Herbst-Bayliss)