From the Cook County Treasurer's website:
"Past years' taxes (from the 2005 tax year or earlier) are the jurisdiction of the Cook County Clerk's Office. These include preparations of bills for open items (property taxes that were not sold or forfeited at a tax sale), forfeitures (property taxes that were forfeited at a tax sale), and redemptions of tax delinquencies that were sold to tax buyers. For this reason, the Clerk's Office generates the message about past years' taxes that appears in the stripe just above the payment coupon on the tax bills.
For information on past years' taxes, have your 14-digit property index number ready and contact the County Clerk's real estate tax services division:
by calling 312.603.5656;
or by visiting the Clerk's Office on the 4th Floor of the Cook County Building, 118 N. Clark St., Chicago, IL 60602. "
You can always check current status by going here: http://www.cookcountytreasurer.com/payment.aspx?ntopicid=3