Respiratory tract infections have been thought to act as a trigger mechanism in sudden infant death. In 118 autopsy cases of infant death, paraffin-embedded or frozen lung tissues were investigated by means of a nested polymerase chain reaction (PCR) to detect adenovirus (AV) DNA. The primers used are general primers and allow the detection of most pathogenic adenoviruses with high specificity and sensitivity and independently of devitalization of viruses or degradation of viral DNA. For the investigation three groups were established: there were 13 cases of unnatural death, 78 cases of natural death without histological signs of interstitial pneumonia, and 27 cases with interstitial pneumonia. The first group was AV negative. In the group without interstitial pneumonia AV was detected in 10.2% of the cases. In the group with interstitial pneumonia the frequency of AV detection was almost 26%. The results obtained demonstrate an association between interstitial pneumonia and detection of AV DNA, indicating that AV may play an important part in pulmonary infection in infants. Histological evidence of interstitial pneumonia was not observed in all AV-positive cases, perhaps because nonspecific virus-related changes occurred only in early stages of viral infection. Comparison of the AV frequency in SIDS (25%) and non-SIDS cases (4%) indicates an association between pulmonary AV infections and sudden death. These results support the working hypothesis of respiratory infections acting as a trigger mechanism in sudden infant death.