The Childhood Asperger Syndrome Test (CAST) is a 37-item parental self-completion questionnaire to screen for autism spectrum conditions in research. Good test accuracy was demonstrated in studies with primary school aged children in mainstream schools. The aim of this study was to investigate the test-retest reliability of the CAST. Parents of 1000 children in years 1-6 in five mainstream primary schools in Cambridgeshire received the CAST. A second identical questionnaire was posted to respondents after approximately 2 weeks. Both mailings generated 136 responses. Agreement above and below a screening cut-point of 15 was investigated. The kappa statistic for agreement (< 15 versus > or = 15) was 0.70, and 97 percent (95 percent CI: 93-99 percent) of children did not move across the cut-point of 15. The correlation between the two test scores was 0.83 (Spearman's rho). The CAST has shown good test-retest reliability, and now requires further investigation in a high-scoring sample.