Preterm infants respond well to the three doses of diphtheria, tetanus, and whole cell pertussis (DTP) vaccine,¹ but dexamethasone treatment may impair immunogenicity.^2,3 We investigated whether four, rather than three, DTP doses may be preferable for primary immunisation of steroid treated preterm infants. Twelve infants born at < 30 weeks gestation who had received dexamethasone for chronic lung disease were given doses of DTP vaccine combined with Hib (ActHIB DTP; Pasteur-Mèrieux-MSD) at 2, 3, and 4 months of age. A fourth dose was administered six weeks after the third immunisation (table 1). With the use …