Chicago Board Options Exchange officials say the systems outage that delayed trading for several hours on Thursday was an internal problem and not the work of hackers.
According to the CBOE web site, users were having problems downloading information from the computer system.
CBOE spokeswoman Gail Osten told NBC 5 that trading resumed on the SPX, SPXpm and SPX Weeklys at 11:50 a.m. and the rest were expected to be back up by noon.
The CBOE is the largest U.S. options exchange. Many of the most frequently-traded stock options are bought and sold on its exchange.