Ninety-seven preterm infants were immunized with diphtheria-tetanus-pertussis (DTP) prior to discharge from hospital. The mean gestational age at birth was 28.1 weeks (range 24-34) and the mean age at immunization was 80.6 days (range 44-257). Nineteen (20%) infants developed apnoea or bradycardia within 24 h of immunization. The infants who developed apnoea and/or bradycardia had a younger gestational age at birth than those who did not (P = 0.03), were artificially ventilated for longer (P = 0.01) and were more likely to have a diagnosis of chronic lung disease (P = 0.006). In the majority of infants these events were not clinically significant. Two infants who developed concurrent upper respiratory tract infections required additional oxygen and one of them was treated with oral theophylline. In general, it is safe practice to immunize preterm infants with DTP unless otherwise contraindicated. However, it is recommended that cardiorespiratory function is monitored after immunization in very preterm infants who had prolonged ventilatory support and/or chronic lung disease.