Cryptic rearrangements involving the telomeres are thought to account for a substantial number of patients with unexplained mental retardation and multiple congenital anomalies, although the exact incidence of these rearrangements is still unclear. With the advent of chromosome-specific telomeric probes and the use of FISH (fluorescence in situ hybridization), it is now possible to identify submicroscopic rearrangements of the distal ends of chromosomes that may otherwise go undetected using conventional cytogenetic studies. We report on a 4 1/2 year-old girl with severe mental retardation and minor anomalies who inherited the unbalanced product of a cryptic translocation involving chromosomes 2 and 17 from her father. The family history was significant for early pregnancy losses, stillbirths, and mental retardation in many other family members, suggesting segregation of a familial translocation. This translocation was detected using chromosome-specific telomere FISH probes, and not visible using conventional cytogenetic methods. Collectively, this case and those previously reported clearly demonstrate the value of a systematic search for cryptic chromosome rearrangements in patients with unexplained mental retardation with previously reported normal chromosome studies; and in particular those with a family history of mental retardation, birth defects, or early pregnancy losses.