Please refer to the IRS Publication 525 Taxable and Nontaxable Income - http://www.irs.gov/pub/irs-pdf/p525.pdf page 16
Amounts you receive as workers' compensation for an occupational sickness or injury are fully exempt from tax if they are paid under a workers' compensation act or a statute in the nature of a workers' compensation act.
The exemption, however, does not apply to retirement plan benefits you receive based on your age, length of service, or prior contributions to the plan, even if you retired because of an occupational sickness or injury.
If your disability pension is paid under a statute that provides benefits only to employees with service-connected disabilities, part of it may be workers' compensation. That part is exempt from tax. The rest of your pension, based on years of service, is taxable as pension or annuity income.
If you retired on disability, you must include in income any disability pension you receive under a plan that is paid for by your employer. You must report your taxable disability payments as wages on line 7 of Form 1040 or Form 1040A until you reach minimum retirement age. Minimum retirement age generally is the age at which you can first receive a pension or annuity if you are not disabled.
Beginning on the day after you reach minimum retirement age, payments you receive are taxable as a pension or annuity. Report the payments on lines 16a and 16b of Form 1040 or on lines 12a and 12b of Form 1040A.
If you receive a disability pension based on years of service, you generally must include it in your income.