There is a lack of data regarding the incidence and clinical significance of apnoea or bradycardia ( AB) following immunisation with combination vaccines containing an acellular pertussis (Pa) component in respiratory stable preterm infants. Medical records of respiratory stable preterm infants who received a first dose of a combined diphtheria ( D) and tetanus ( T) toxoids, Pa, Haemophilus influenzae type b (Hib), inactivated poliovirus (IPV) vaccine with or without hepatitis B virus (HBV) in the neonatal intensive care unit (NICU) of the University Children's Hospital Basel between January 2000 and June 2003 were analysed. For each infant, clinical data were recorded for a 72 h period before and after immunisation. Of 53 infants with a mean gestational age of 28 weeks, 7 (13%) showed a transient recurrence of or increase in episodes of AB following immunisation. Five of these seven infants required intervention ranging from tactile stimulation to bag and mask ventilation. Regarding risk factors, children with recurrent or increased AB were indistinguishable from those without such events. The rate of fever (> 38 degrees C) following immunisation was higher in affected infants compared to those without recurrence of or increase in AB (3/7 vs 2/46, P = 0.01). Conclusion: Although most infants tolerated immunisation well, the incidence of recurrent or increased apnoea or bradycardia in respiratory stable preterm infants following the first immunisation with penta- or hexavalent vaccines was 13%. Most apnoea or bradycardia events required intervention but did not have serious consequences. Monitoring of all preterm infants following immunisation in the neonatal intensive care unit is recommended.