Infants with birth weights under 750 g are disproportionately represented in perinatal mortality and morbidity rates. We reviewed the outcomes of 98 infants delivered at our perinatal center between July 1982 and June 1985 (period 1) whose lengths of gestation were 20 or more weeks and whose birth weights were under 750 g, and compared them with the outcomes of 129 such infants born between July 1985 and June 1988 (period 2). The frequency of cesarean section increased from 12 to 19 percent between the two periods. During the entire six-year period, 12 percent of the infants with birth weights under 500 g were intubated, as compared with 28 percent of those between 500 and 599 g, 60 percent of those between 600 and 699 g, and 90 percent of those between 700 and 749 g. The frequency of endotracheal intubation increased between the two periods only for infants with birth weights above 500 g (P less than 0.02). Despite more aggressive treatment, survival did not change, although the mean time to death among infants transferred to the neonatal intensive care unit increased from 73 to 880 hours. Among all live-born infants with birth weights under 750 g, the rate of survival was 20 percent during period 1 and 18 percent during period 2, but 48 and 43 percent of those transferred to the neonatal intensive care unit survived in the two periods reviewed. Neonatal morbidity also did not change. Among survivors at a corrected age of 20 months, 4 of 18 born during period 1 and 7 of 14 born during period 2 had moderate-to-severe neurodevelopmental impairment. When all live-born infants of less than 28 weeks' gestation were considered, only 8 percent of those born at 23 weeks survived, as compared with 16 percent of those born at 24 weeks, and 53, 63, and 72 percent of those born at 25, 26, 27 weeks, respectively. Thus, despite a tendency to perform more cesarean sections and active resuscitations, no improvement in the survival of babies with lengths of gestation below 25 weeks or birth weights under 750 g was observed. The probability of survival is very poor if the length of gestation is less than 24 weeks or the birth weight less than 600 g.