This research study examined the relationship between demographic, achievement, and socioeconomic factors and differences in proportions of students served in special education in school districts and intermediate education agencies (AEAs) in Iowa. Vast differences were found in the proportions of students being served in special education programs, ranging from 3% to 17% of school district populations. Four research questions were addressed in the study: (a) Are demographic, achievement, and socioeconomic factors related to the proportion of students served in special education in school districts? (b) Are demographic, achievement, and socioeconomic factors related to the proportions of students served in special education in area education agencies (AEAs)? (c) Is the relationship between demographic, achievement, and socioeconomic characteristics and the proportion of students with severe disabilities different from the relationship of these same factors and the proportion of students with mild/moderate disabilities? (d) Can a procedure be developed that would assist in predicting the proportion of a school district's population that should be served in special education?;The specific measures of demographic, achievement, and socioeconomic characteristics used in the study were: district size, population density, pupil-teacher ratios, percentage of adults lacking high school diplomas, Iowa Test of Basic Skills scores, average gross income, property values, and percentage of students receiving free and reduced price lunches;Although significant correlations were found between six of the eight variables and the proportion of students in special education, the combined relationship of all these variables accounted for only 17% of the variance in proportions of students. Relationships among the variables to proportions of special education students appeared highly idiosyncratic in the AEA analysis with no observable pattern identified. Four of the eight factors correlated more strongly with proportions of students served in programs for mild/moderately disabled than proportions of students served in programs for severely disabled. A prediction equation was developed to determine the proportion of students who should be served in special education in each school district in Iowa based on demographic, achievement, and socioeconomic variables;Findings in this study were consistent with prior research suggesting relationships between the variables tested and proportions of students in special education. However, results would indicate that the relationships between the variables and the proportions of students in special education were not of a magnitude that would justify the current range in proportions of special education students. Further analysis in the form of case studies at the school district and building level is needed in order to more fully understand the present variation among school districts in the proportions of students in special education.