A nationwide survey on the epidemiology of chronic lung disease (CLD) of the newborn was conducted. Questionnaires were sent to 391 level II and III neonatal centers in Japan and the registration of infants born in 1990 with chronic lung disease was requested. CLD was defined as an oxygen requirement greater than that obtainable in room air at 28 days after birth, with symptoms of persistent respiratory distress and a hazy or emphysematous and fibrous appearance on chest X-ray. A total of 301 neonatal centers (77.0%) responded and 50,290 infants at these centers were registered. Of these, 97% survived the first month and 1,135 of 48,762 neonatal survivors developed CLD. The mortality of infants with CLD was 6.2%. Survival rates at 28 days of age increased consistently with birthweight. Survival at 28 days of age in infants below 1,000 g at birth was 73.7%, but the rate was 93.9% in infants weighing 1,000-1,499 g. The incidence of CLD was inversely proportional to birthweight. Approximately one quarter of neonatal survivors with a birthweight below 1,500 g and approximately half of extremely small infants ( < 1,000 g) developed CLD. The analysis of CLD infants showed that 28.2% of them had a history of respiratory distress syndrome (RDS) and a typical fibrous appearance on chest X-ray (Type I), while 29.3% also had a history of RDS but had an atypical X-ray appearance (Type II).(ABSTRACT TRUNCATED AT 250 WORDS)