Babies of 37-41 wk gestation are, by international convention, said to be born at 'term', but some still develop respiratory distress. It is not clear how mature a baby has to be to be free of risk of primary surfactant deficiency. An area-based retrospective study of all the 179,701 babies of 34 or more weeks' gestation born alive in a defined area of the north of England in 1988-92 identified 149 babies with features of respiratory distress typical of surfactant deficiency severe enough to be managed with ventilatory support and with no evidence of aspiration or intrapartum infection. Gestation was carefully cross-validated against antenatal information, including at least one ultrasound assessment in the first half of pregnancy. Thirty-six of these babies were born at or after 37 wk gestation. Only 4 of the 35 delivered at 37-38 wk went into spontaneous labour. Seven became ill enough to be candidates for ECMO and two died. A review of all neonatal deaths in the study area between 1981 and 1995 identified four similar deaths in 1981-87 and two in 1993-95. Babies who are not premature, using the internationally agreed definition, can show signs of potentially lethal pulmonary immaturity at birth, especially if subjected to pre-labour Caesarean delivery. Those born at 37-38 wk are 120 times more likely to receive ventilatory support for surfactant deficiency than those born at 39-41 wk. Elective delivery should only be undertaken before 39 wk gestation for good medical reasons.