Revenue fell 2.4 percent to $16.15 billion, below analysts' estimates of $16.26 billion, according to Thomson Reuters I/B/E/S.
Comcast's cable unit, which has the most U.S. subscribers of any cable company with 21.6 million video customers, missed estimates in some key areas.
Comcast lost about 129,000 video subscribers in the quarter, which was worse than the loss of 102,000 subscribers Wall Street analysts were expecting, according to Street Account.
Comcast, like its smaller rivals, increasingly relies on Internet customers for growth as they continue to lose cable TV subscribers to competitors such as Verizon FiOS and AT&T U-verse and grapple with rising programming costs. Comcast added 297,000 high-speed Internet customers, which slightly missed estimates of a gain of 305,400 customers.
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The cable industry faces a long-term challenge from customers who consume an increasing amount of Internet video and subscribe to lower-cost alternatives such as Netflix.
Operating cash flow at its cable unit increased 6.2 percent to $4.24 billion.
Comcast clinched full control of NBC Universal for $16.7 billion earlier this year by buying out General Electric's stake a few years ahead of schedule. It owns broadcaster NBC, the film studio Universal and a host of cable channels.
In its NBC Universal business, its biggest cash flow growth was seen at its filmed entertainment unit, where it generated operating cash flow of $189 million compared to $72 million a year ago thanks to the hit kids movie "Despicable Me 2." Total operating cash flow at NBC Universal rose 9.6 percent to $1.25 billion.
Shares in the U.S. cable giant, which also owns CNBC parent NBC Universal, rose before the opening bell, following the news. (Click here to get the latest quotes for the company.)