Many big cities have instituted systematic retention to replace social promotion policies. This study examined the extent to which retaining low achieving students in their current grade for an additional year has a built-in effect on the average reading scores for the grade in which they were retained and the grade they would have been promoted to. Reading achievement means were calculated with and without the inclusion of non-promoted students at one elementary school in Kansas City, Missouri. The analysis indicated that the achievement scores can increase in both the retention grade and the next higher grade when low achievers are not promoted. Some of the findings are spurious since they may reflect only continuing poor performance by the low achievers who were retained. Therefore, such assessments of achievement outcomes should disaggregate the scores of promoted and retained students and should include calculation of annual increase or decline for each group at each grade level. An important effect of retention was that teachers' opportunity for delivering effective instruction at a grade level was enhanced. Given the concern that exists regarding achievement levels in big city schools, decision makers should assess and consider the effects, both spurious and otherwise, of the promotion policies in these schools. Statistical tables are included. (VM)