The present study was undertaken to clarify how we should assess the necessity of close follow-up in each case, when we first examine an infant with ankle clonus within the first year of life. The neurologic prognoses of 169 infants who had exhibited ankle clonus at least once during the first year of life were reviewed in relation to the age at examination for positive response, degree of response, and coexisting neurologic signs other than ankle clonus. Ninety-seven of them (57.4%) were normal, while the rest had diverse pathologic outcomes: cerebral palsy in 49, mental retardation in 12, borderline intelligence in 9, and motor delay in 2. The neurologic prognoses in infants whose ankle clonus was more than ten beats at any age within the first year of life, and ten or less beats over 8 months of age were always abnormal. In the subjects whose neurologic signs other than ankle clonus appeared within 4 months of age and persisted after 5 months of age, prognoses were generally poor. In infants with such findings, their clinical courses should be closely observed for the possible development of neurologic abnormalities.