Interstitial deletions of the long arm of chromosome 4 are rare. The deletions may occur at the proximal or the distal portions of the chromosome and different breakpoints may be involved. We report an interstitial deletion of 4q: 46XY der 4 (q28;q35) in a six-year-old boy with dysmorphic features associated with moderate mental retardation. Parental chromosomal analysis showed a balanced paternal translocation.