Summer school is thought to be an effective alternative delivery mode for Chapter 1 education. The effectiveness of summer Chapter 1 programs was studied during Fall 1993 in 68 Chicago (Illinois) public schools. Students who had been in fourth grade in 1991-92 were divided into a group that had received Chapter 1 help during the school year and had attended 1992 summer school programs (n=1,006) and a group that had only school-year instruction (n=490). Reading comprehension and mathematics scores on the Iowa Tests of Basic Skills were used as achievement pretest and posttests. Advanced-skill instruction during the summer, received by 407 of the students, was positively and significantly related to students' adjusted yearly math and reading gains. Simply attending summer school did not help the most educationally disadvantaged students catch their Chapter 1 peers. (Contains one figure, one table, and nine references.) (SLD)