Partial trisomy of the long arm of chromosome 4, usually resulting from a familial segregation of a balanced translocation, has been described in a number of patients. This report describes the genetic and endocrine findings in a 16 year old 46,XY,12q+ mentally retarded male. The banding pattern of the extra chromatin material from this de novo unbalanced translocation shows that the distal segment of the long arm of chromosome 4 is involved. Comparison of the clinical features in this patient with cases of partial trisomy 4q previously reported support the cytogenetic evidence for this translocation involving the distal portion of 4q. Endocrine data suggested an end-organ resistance, characterised by extreme hyperinsulinaemia, primary hypothyroidism, and hypergonadotrophic hypogonadism associated with no signs of autoimmunity. To our knowledge, no endocrine evaluation has been previously reported in patients with partial trisomy 4q.