A 2 1/2-year-old female is reported with ring 18 chromosome syndrome. The chromosomal abnormality was found in all examined leukocytes and cultured skin fibroblasts. Besides the usual clinical characteristics of this syndrome, two additional features are described that have not been reported previously: right hemidysmorphism, including hypertrophy of the tongue and lower extremity; coloboma of the lower right side of the gums; atretic external right ear canal; and hypotonia with mitochondrial encephalomyopathy associated with excessive ketonemia during normal food intake and a large increase after overnight fast.