In a 3¾ year old boy with mental and physical retardation, a chromosome analysis from lymphocyte cultures revealed a partial deletion of chromosome 1q following a pericentric inversion. The chromosomes of the parents were normal. The clinical picture of the patient included the following characteristics: prominent occiput, small chin, deep-seated and dysplastic ears, abnormal vortices of the hair, divided tip of the tongue, high palate, small finger and toe nails, inguinal hernia both sides, undescended but normal sized testes, hypotonic musculature and overextensible joints, retardation of ossification in the left hand by 6-12 months, slight osteoporosis, EQ~0.5.