The viability limit defined by the Japanese Eugenic Protection Act was amended from 24 to 22 completed weeks of gestation in 1991. To testify if the amendment is appropriate, we conducted a survey on the mortality and morbidity rates of infants less than 600 g born in Japan between 1984 to 1993. Questionnaires were mailed to 205 hospitals with neonatal intensive care units (NICUs) and 165 (80%) responded. Of 1655 infants <600 g birth weight and admitted to the NICUs included in this survey, 457 (28%) survived to hospital discharge. The survival rates of infants born <24 weeks and >==24 weeks of gestation were 17% (128/748) and 36% (329/903), respectively; and of infants <500 g and 500 to 599 g at birth were 16% (82/510) and 32% (375/1145), respectively. None of the infants <==20 weeks of gestational age and <==350 g at birth survived, but 4% (2/49), 12% (27/218), 21% (99/474), and 34% (131/381) born at 21, 22, 23, and 24 weeks of gestation survived, respectively. The majority (68%) died within 1 week after birth and only 10% died after the neonatal period. The main causes of death were: acute respiratory failure (33%), intraventricular hemorrhage (20%), infection (16%), and heart failure (10%). Of 457 survivors, 65% were free from handicaps. The incidence of mental retardation (DQ < 70), visual disturbance, and CP were 15%, 14%, and 11%, respectively. Admission of micropremies to NICU increased markedly after the amendment of the Eugenic Protection Act, despite a marked decline in birth rate. The survival rate increased from 22% to 33% after generalized use of surfactant in 1988, but the handicap rate (35%) among survivors remained unchanged. The new viability limit of 22 complete weeks of gestation was feasible, since survival of less than 22 weeks was exceptional while survival of 22 to 23 weeks was 18%.