Referential communication was examined in youth with Down syndrome or fragile X syndrome in comparison to each other and to MA-matched typically developing children. A non-face-to-face task was used in which the participant repeatedly described novel shapes to listeners. Several dimensions of referential communication were especially challenging for the syndrome groups (i.e., they displayed below-MA performance), although there were differences in the dimensions that each syndrome group found to be most challenging. Independently assessed expressive language ability contributed to variations in referential performance, especially for participants with Down syndrome.