Perinatal factors related to the incidence of respiratory distress syndrome were analyzed by the multiple logistic regression statistical method in 263 mothers and their 298 offspring delivered between 24 and 35 weeks' gestation in a 1-year period in a regional referral perinatal center. The risk of respiratory distress syndrome in white infants rose with decreasing gestational age (p less than 0.0001) while prolonged rupture of membranes of greater than 24 hours in the absence of maternal infection (28% of cases) was highly protective (p less than 0.0001). Compared with vaginal delivery, cesarean delivery without labor increased the risk of respiratory distress syndrome (p = 0.03). The administration of tocolytic drugs was unrelated to the incidence of respiratory distress syndrome, but corticosteroid therapy given at least 72 hours before delivery was protective (p = 0.03). Male and female infants were equally at risk for respiratory distress syndrome as were black and white infants, but other races had a lower incidence (p = 0.004). Infants with respiratory distress syndrome were on mechanical ventilators longer than those with other respiratory illnesses.