Two males with apparently identical rings of chromosome 7 and very different phenotypic features are presented. Patient 1 is severely retarded, has short stature, microcephaly, craniosynostosis, unilateral proptosis, ptosis, and microcornea, small phallus, first-degree hypospadias, and undescended testes. Patient 2 is of normal intelligence, and his only abnormalities are short stature and a small head. Study of lymphocytes at metaphase showed the same proportion of cells with the ring, 46, XY, r(7), and those without a ring, 45, XY,-7, in each case. Quinacrine fluorescence and Giemsa banding techniques showed all the bands of chromosome 7 to be present, suggesting little material missing as a result of ring formation. A similar degree of instability of the ring in each case was indicated by the frequency of abnormal cells at anaphase and telophase.