Animal experiments and observations on quantitative growth of human cerebellum suggest a critical period when its development is particularly vulnerable to hypothyroidism. Sixty-seven patients aged 7-24 years with hypothyroidism under long-term treatment were examined for ataxic symptoms. These were found in 24 of 39 patients (60%) hypothyroid before or during the third month of life. Only 4 of 18 patients (20%) hypothyroid later had cerebellar symptoms. Such symptoms could be evidence for the onset of hypothyroidism before or during the 3rd month of life. Seventeen (80%) of mentally retarded patients had cerebellar symptoms compared with 11 (30%) of 45 attending normal school. Even retrospectively, these data might permit a more accurate prognosis of further mental development in hypothyroid children.