As a service to healthcare professionals, Aventis Pasteur MSD UK Ltd. operates a telephone-based Vaccine Information Service, providing information on all aspects of vaccination. In the UK it is the primary means by which spontaneous adverse drug reaction reports are received by the company. It was brought to the attention of the Pharmacovigilance Department that a significant number of calls related to people seeking advice following inadvertent administration of vaccines. To inform our advice it was decided to collect details of such episodes, to enquire whether an adverse drug reaction had already occurred, and to encourage reporting of adverse drug reaction that may occur subsequently. Inadvertent vaccination during pregnancy was not included in this survey since these data were already being collected separately. During the period from 1 September 1999 to 31 August 2000 the Vaccine Information Service received 124010 enquiries. Of these, 302 (0.2%) concerned inadvertent administration of one or more vaccines (all age groups), 161 (53.2% of total inadvertent administration) of them in children (<18 years). These 161 reports involved the inadvertent administration of 221 vaccines. In six cases (3.8%) one or more adverse drug reaction were reported following the inadvertent administration. Five of these six cases involved a DTP-containing vaccine: one case where DTP was given instead of diphtheria and tetanus toxoid (DT) vaccine as a pre-school booster, one case where a fourth dose of DTP-Haemophilus influenzae type b conjugate vaccine (Hib) was given at 20 weeks of age and three cases where DTP was mixed with DTP-Hib. The sixth case involved a child given an adult dose of hepatitis B vaccine. Data are available for five of these six cases-all adverse drug reactions were non-serious and resolved without sequelae.Inadvertent administration of vaccines in childhood, although worrying for both the healthcare professionals and the parents involved, seems rarely to result in adverse reactions.