(MONEY Magazine) – The recent IRS decision to define obesity as a disease means that certain costs of losing weight may now be deductible. To take a write-off, you must have a doctor's diagnosis that you are obese or must lose weight to treat a condition such as diabetes or hypertension. Not all costs will qualify. "Certain treatments are sure to be deductible, such as stomach-stapling or Weight Watchers fees," says Martin Nissenbaum, director of personal income tax planning at Ernst & Young in New York City. Other likely deductions: fees for an aerobics class, a personal trainer, exercise equipment and perhaps a portion of a spa stay. The IRS suggests including a doctor's note with your return. You cannot deduct gym membership dues or diet foods. Like all deductible medical expenses, only those that exceed 7.5% of your adjusted gross income qualify. The rule is retroactive, so you can amend any return from the past three years to claim the deduction. --JUDY FELDMAN