We report the first case of identical female twins who satisfy the proposed diagnostic criteria for Cohen syndrome. The sisters presented with retinal degeneration, obesity and mental retardation, and had the characteristic facial appearance. The manifestations of previously reported cases of Cohen syndrome are reviewed. Unusual changes in our patients include tall stature, macrocephaly, and transient cardiomyopathy during the first year of life. These anomalies have been reported previously in other patients with Cohen syndrome, and suggest that the disorder is phenotypically heterogeneous. Precocious puberty was present in both girls; the latter findings have not been reported previously in the Cohen syndrome. Detailed metabolic and cytogenetic analysis demonstrated no abnormalities.