After delaying its open due to "downloading" issues, the Chicago Board Options Exchange reopened trading on all its products at 1:00 p.m. ET.
Delay, which the CBOE said was the not the result of hacking, prevented investors from accessing its exclusive options on the Standard & Poor's 500 Index and the CBOE Volatility Index.
Trading continued on the 10 other U.S. options exchanges.
At least two of CBOE's competitors invoked "self-help" against CBOE, an action taken when another exchange is having system problems.
The Chicago exchange, run by CBOE Holdings, is the oldest U.S. options market. Most trading of options on individual stocks takes place by computer screen, but dozens of open-outcry traders crowd the areas of the CBOE floor dedicated to CBOE's exclusive stock-index options.
"Some users are experiencing issues downloading CBOE and CFE products this morning. The CBOE is looking into this issue and will advise," the CBOE said on its website before its regular opening time of 9:15 am local time. CFE is CBOE's futures market.
Later CBOE said systems for all its exchanges except its floor-based Chicago options exchange were functioning normally.
At least one trader was unruffled.
"There is no shortage of other venues to trade on," said Steve Sosnick, equity risk manager at Timber Hill, a division of Interactive Brokers Group. "We have not heard much about the cause of the outage at CBOE."
The SEC is "monitoring the situation" a spokesman said.