A standardized assessment of autistic symptomatology was completed for 29 children and young adults with a supernumerary isodicentric chromosome 15 (formerly known as inverted duplication 15). Although there was variability in severity, 20 individuals with an isodicentric chromosome 15 [idic(15)] had a high probability of being autistic. Eight of the 9 remaining children were under age 5 years and were more sociable than the rest of the cohort. Group characteristics such as gender and seizure presence could not explain the observed difference between older and younger individuals in our study. The natural history of isodicentric 15 syndrome remains to be shown through longitudinal work and may include an age-related risk for developing autism.