To establish the enzyme defect in gyrate atrophy, we measured the activity of ornithine aminotransferase in phytohemagglutinin stimulated lymphocytes in a patient with gyrate atrophy, her daughter, and three normal controls. The patient's cells had no detectable ornithine aminotransferase activity and the daughter's cells had 44% of control activity. This intermediate value is characteristic of an obligate heterozygote. These results are the first demonstration of an enzyme defect in gyrate atrophy.