A cryptic translocation t(5;18)(qter;qter) was detected in a large family, using a FISH-based approach combining subtelomeric probes to allow the subtelomeric regions of most chromosome ends to be analysed for deletions and balanced or unbalanced translocations. Unbalanced karyotypes (duplication 5qter/deficiency 18qter) resulted in a previously undescribed association of moderate to severe mental retardation, microcephaly, pre- and postnatal growth retardation, distinct facial dysmorphism, narrow auditory canals, genital hypoplasia, left heart hypoplasia in one patient and severe behaviour difficulties in another. Some of the features observed in affected individuals are characteristic of known syndromes involving either 18q (growth deficiency, nystagmus, narrow auditory canals, genital hypoplasia, behaviour problems in 18q deletion syndrome) or 5q (umbilical and inguinal hernias, congenital heart defects in distal 5q trisomy).