"While strong, this year's retail sales results since the Thanksgiving weekend have not been at the pace we had anticipated," CEO Mel Karmazin said in a statement. The company said it now expects to end the year with between 5.9 million and 6.1 million subscribers.
The satellite radio firm said in November that it expected to have 6.3 million subscribers by year's end.
Sirius still hopes to post its first quarter with positive free cash flow, after capital expenses, as early as this quarter, the company said.
In after-hours trading, shares of Sirius were trading at $3.99, a drop of 18 cents, or more than 4 percent, from where the stock ended regular trading.