A 7-year-old boy with mental retardation had apparently balanced reciprocal translocations, involving the telomeric regions of chromosomes 1p and 4q, which was detected by routine chromosome analysis. Fluorescence in situ hybridization (FISH) was used and also revealed the telomeric region of chromosome 16p to be involved in a still apparently balanced translocation-complex, impossible to discover with classical cytogenetic analysis. We want to emphasize the importance of FISH in detecting small chromosomal aberrations. We discuss whether the abnormal phenotype is caused by unbalanced karyotype with cryptic undetected translocations or small deletions or mutations in the translocation-breakpoints.