A 21-month-old male infant with 46,XY,r(21) constitution identified by G and R banding is reported. The main clinical features were mental and physical retardation, microcephaly, antimongoloid slant of eyelids, malformed and low set ears. The clinical and cytogenetic findings of previously reported cases of r(21) are reviewed. Clinical resemblance to the eleven cases described in the literature is striking. The assays for superoxide dismutase (SOD-1) activity in the red blood cells from patient and his parents gave normal results. This findings is compatible with the hypothesis that only the segment distal to the SOD-1 locus, i.e. 21q22.2 leads to qter, has been lost during ring formation.