Lymphocyte chromosomes of untreated and thyrotoxic patients treated with radioactive iodine have been compared with those of control subjects studied concurrently. Untreated patients had a higher percentage of cells with more than 46 chromosomes. 131I-treated patients who were compared with controls within 2–3 months of therapy had significantly increased incidences of cells containing fewer than 46 chromosomes and cells with chromatid and chromosome aberrations. Patients examined 2–5 yr. after radioactive iodine treatment showed similar changes in lesser degree.