A de novo 11p+ chromosome was found in a child with mild mental retardation but no other remarkable dysmorphic characteristics. Banding studies suggested a duplication of regions 11p13 and 11p14 or regions 11p14 and 11p15. Using fluorescent in situ hybridization and digital imaging microscopy, we mapped probe p32.1 (D11S16) to the proximal part of region lip 14 (lip 14.1) and demonstrated duplication of this probe in our patient. Southern hybridization showed duplication of p32.1 and other probes located at 11pl 3 and 11ρl4, but the gene for α calcitonin (CALCA), located at 11p15, was not duplicated. The application of these techniques led to the identification of the duplication as dir dup(11 )(pter→p 13::p 15.1→