Objective We reevaluated 49 girls with either Rett syndrome (RTT) or features of RTT who had negative test results for mutations in the MECP2 gene and compared them with 49 girls who had positive test results. The girls with MECP2-positive results included 2 girls with forme fruste and 2 with congenital RTT. Study design Based on the original diagnostic criteria for RTT, we developed a 10-item checklist with a score ranging from 0 to 12. Results If only girls with a score of 8 or more had been tested, 46% of the girls without mutations would have been excluded from testing without missing a single girl with MECP2-positive results. Conclusions This checklist provides a simple aid for deciding whether or not a genetic test for RTT should be performed with only a minimal risk of missing girls with MECP2-positive results. (J Pediatr 2003;142:332-5)