The hospital records of 197 infants with the respiratory distress syndrome (RDS) were reviewed and the families of 111 of them subsequently contacted to obtain a family history. After correcting for biasis of ascertainment, the incidence of RDS among the full sibs was found to be between 12 and 19% depending on whether the individuals diagnosed as "possible RDS" were counted as affected. Among the low birth weight (LBW, less than or equal to 2.5 kg) and/or preterm (less than or equal to 37 weeks gestation) infants in the sibships, the incidence of RDS was 32-50%. Considering only sibs born after the probands yielded the empiric recurrence risk of 17--27% for all younger sibs and 39--67% for LBW/preterm younger sibs. The risk for maternal half-sibs was of about the same magnitude as that for full sibs, while the risk for paternal half-sibs was minimal. Among the LBW/preterm first cousins of probands, only the infants of maternal aunts showed an RDS incidence clearly higher than that in the general population. We think these data suggest a genetically determined maternal factor predisposing the infants of certain mothers to RDS. Other significant findings include: 1) an excess of males among the probands but a normal sex ratio among the sibs of the probands; 2) a decrease in mean birth weight and mean length of gestation for not only the probands but also their sibs; 3) a decrease in the mean parental ages at the birth of the probands; 4) a relative dearth of first-born and an excess of second-born infants among the probands; 5) an increased incidence of stillbirths in the sibships; 6) an increased number of probands born by cesarean section; and 7) a twin concordance of 75%.