A moderately mentally retarded 10-year-old boy of very short stature was found initially to have a complex chromosomal rearrangement (CCR) involving chromosome 1, 2, 3, 4, and 8. A balanced twelve-breakpoint CCR was suggested after extensive investigations including subtelomere FISH, whole chromosome paints, comparative genomic hybridization (CGH), multicolor FISH (MFISH), and spectral karyotyping (SKY). SKY and MFISH gave slightly discrepant results. For further clarification of the karyotype, multicolor banding (MCB) analysis and FISH with region-specific YAC probes were done. This allowed clarification of a sixteen-fragment CCR to be made, the most complex constitutional chromosomal rearrangement reported so far. Remarkably, two 'secondary' insertions originated from the interior of a 'primary' insertion by an excision/duplication event. The randomness of the fragments and the complexity of the derivative chromosomes suggest that this CCR is the result of a single meiotic event, e.g., faulty repair of a five-chromosome knot.