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Your actual loss for tax purposes is not your mortgage payment minus rent---it is a little more complex than that. You are allowed to deduct your mortgage interest, property taxes, fees, most expenses, etc. and deprectiation for the property. Most persons who own rental real estate have losses in the first few years especially.
For tax purposes, in general you are allowed a loss of up to $25,000 per year as a real estate, passive loss (less if you file married separately) but there is an income limitation to the loss that you are allowed to take. If your Adjusted gross income is greater than 100K, $1 of your loss is disallowed for every $2 over 100K that you earn. This means that if your income was 150K, your loss would not be able to be taken on the current return. However--your loss carries forward, so that when you have other passive income or you have less overall income (or you sell the property) you can take advantage of this loss.
Here is more detail: http://www.irs.gov/taxtopics/tc414.html
page 13 in this pub for loss limits: http://www.irs.gov/pub/irs-pdf/p527.pdf
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