The causal effects of a voluntary summer reading intervention on children's reading activities and reading achievement were assessed in a randomized experiment involving 331 children in Grades 1-5. Children were pretested in the spring on a standardized test of reading achievement (Stanford Achievement Test, 10th ed.), on the Elementary Reading Attitudes Survey, and on a reading preference survey. At the end of the school year, children were stratified by their grade level and classroom and were randomly assigned to receive 10 books matched to their reading levels and preferences during summer vacation or after the administration of posttests. Children in the treatment group received books through airmail in July and August. In September, children were readministered the reading test and completed a survey of their summer reading activities. Although the treatment group reported reading more books and participating in more literacy activities than did the control group, there was no significant difference in reading achievement. Recommendations for enhancing the effects of voluntary reading through teacher-directed instruction and for conducting a replication study are discussed.