To provide the first interstate quasi-experimental assessment on the power of content integration in building reading skills, the authors studied the effects on reading comprehension of GeoLiteracy--a K-8 package of 85 lessons that teaches geography in the context of practicing reading and writing skills. Ninety-six third through eighth grade teachers in Arizona and Michigan divided up into intervention and comparison groups. Intervention teachers taught GeoLiteracy lessons during their language arts or social studies times, and their comparison teacher counterparts taught the curriculum as usual--without GeoLiteracy. Statistical analyses of reading comprehension assessments of the 2,539 students involved reveal statistically significant improvement in reading comprehension scores for students in Grades 5 through 8 who were taught using the GeoLiteracy curriculum. Qualitative data regarding intervention teacher practice are also described. (Contains 3 tables, 3 figures and 5 notes.)