The following from http://www.irs.gov/taxtopics/tc419.html
There are more links embedded within this text at the IRS's web sit, if you want to read further.
If you have sufficient income to bet enough to win 15K in one sitting, you probably have a CPA do your taxes. If not, you should!
Topic 419 - Gambling Income and Expenses
Gambling winnings are fully taxable and must be reported on your tax return. You must file Form 1040 (PDF) and include all of your winnings. Gambling income includes, but is not limited to, winnings from lotteries, raffles, horse races, and casinos. It includes cash winnings and also the fair market value of prizes such as cars and trips. For additional information, refer to Publication 525, Taxable and Nontaxable Income.
A payer is required to issue you a Form W-2G (PDF) if you receive certain gambling winnings or if you have any gambling winnings subject to Federal income tax withholding. All gambling winnings must be reported irrespective as to whether any portion thereof is subject to withholding. in addition, you may be required to pay an estimated tax on your gambling winnings. For information on withholding on gambling winnings, refer to Publication 505, Tax Withholding and Estimated Tax.
You may deduct gambling losses only if you itemize deductions. Claim your gambling losses as a miscellaneous deduction on Form 1040, Schedule A (PDF). However, the amount of losses you deduct may not be more than the amount of gambling income you have reported on your return. It is important to keep an accurate diary or similar record of your gambling winnings and losses. To deduct your losses, you must be able to provide receipts, tickets, statements or other records that show the amount of both your winnings and losses. Refer to Publication 529, Miscellaneous Deductions, for more information.