You owe income tax on all your winnings, and if you itemize your deductions, you can deduct losses. If audited, you may be required to prove the amount of your winnings and the amount of your losses. The fact that the casino did not issue the correct paperwork does not mean the winnings are tax-free. It might make it harder for the IRS to catch you, but if they do catch you for under-reporting a six-figure income, the penalties and late interest payments will be substantial.
