In 1985 twin boys simultaneously succumbed to sudden unexpected deaths two to three hours after vaccination with diphtheria, tetanus, and pertussis vaccine (DTP). This occurrence again raises the question of whether an association of sudden infant death (SID) with vaccination is other than temporal. Taking the incidence of SID in conjunction with rates of infant vaccination in the United Kingdom, nine infants would be expected to die, each year by chance alone, suddenly within 24 hours of (and within each 24 hour period succeeding) vaccination with DTP. Twins are at a greater risk of SID than single born infants and occasionally are found dead together. A number of studies into DTP vaccination as a risk factor in SID have shown that SID is less common in vaccinated than in unvaccinated infants.