That is required to calculate the non-taxable part. Only that amount will not be taxable on a pro-rated basis.
See IRS Pub 575 page 10
https://www.irs.gov/pub/irs-pdf/p575.pdf
In general, your cost is your net investment in the con-tract as of the annuity starting date (or the date of the dis-tribution if earlier). To find this amount, you must first fig-ure the total premiums, contributions, or other amounts you paid. This includes the amounts your employer con-tributed that were taxable to you when paid. However, see Foreign employment contributions, later. It doesn't include amounts withheld from your pay on a tax-deferred basis (money that was taken out of your gross pay before taxes were deducted). It also doesn't include amounts you con-tributed for health and accident benefits (including any additional premiums paid for double indemnity or disability benefits).
Thanks for your prompt response. There was no investment on my part for this annuity. All was funded by my Employer - old school pension plan! No monies were withheld from my pay to fund this. I am expecting that this monthly retirement payment to me would be 100% taxable income?