The equity of public school funding in Illinois is investigated in this report. A longitudinal, cartographic methodology examines the county as the unit of analysis for the school years 1972-73 through 1990-91. Findings indicate that Illinois school districts are currently more unequal than when the state equalization aid formula state was adopted in 1970. A dramatic reduction in inequalities 3 to 4 years after implementation show that the equity situation improves as the proportion of state dollars increases and the proportion of local district support decreases. Also, more districts' expenditures are dependent on district wealth. Causes contributing to financial disparities include: (1) the unequal economic development of various regions within the state; (2) the grant-in-aid system in effect until 1980; and (3) a reduction in state aid. Recommendations call for examining longitudinal records other than expenditures per student and balancing resources among districts. Appendix A contains seven tables, Appendix B includes seven cartographic figures, and Appendix C details the computation of the Gini coefficient. (18 references) (LMI)