The first U.S.-based exchange-traded fund hit a major milestone on Thursday, following recent all-time highs for the fund tracking the S&P 500.

What Happened:Launched in January 1993, theSPDR S&P 500 ETF Trust(NYSE:SPY) is the oldest U.S.-based ETF and the largest by assets under management.

The ETF reached the $600 billion assets under management milestone Thursday, according to data from ETFDB.

Managed byState Street Corporation(NYSE:STT), the ETF was launched with seed funding of $6.5 million after being approved by the U.S. Securities and Exchange Commission in 1993, after several years of...