Premature children (n = 25) with respiratory distress (RD) were studied regarding complement activation and formation of the anaphylatoxins C3a and C5a. Blood samples were drawn on admission to the paediatric intensive care unit. In 18 of the patients RD was accompanied by other perinatal complications like pneumothorax or intracerebral haemorrhages. Seven of the premature children had RD without such complications. Preterm children with RD and with peri- and postnatal complications such as pneumothorax or intracerebral haemorrhage had increased concentrations in plasma of the anaphylatoxins C3a and C5a compared with preterm children with RD without these complications. There was a positive correlation between the plasma C3a and C5a concentrations in the preterm children. CONCLUSIONS The present study indicates that isolated RD will appear without signs of complement activation and that complications like pneumothorax or intracerebral haemorrhages are associated with release of the anaphylatoxins C3a and C5a.