Chlamydia inclusions could be demonstrated by an immunofluorescence assay in formalin-fixed lung sections in 32 of 166 cases (19.4%) of Sudden Infant Death Syndrome (SIDS) and in the lungs of only 1 of 30 infants with a known cause of death (3.3%). The difference is statistically significant (P = 0.04). Chlamydia trachomatis is an agent of pneumonia in 1-4 month-old infants who have acquired the disease from an infected cervix during birth, but other chlamydia species are also capable of causing pneumonia. The lung sections of the 32 chlamydia positive SIDS cases did not show typical histological signs of pneumonia. Even though chlamydia inclusions were detected in the lungs of 32 SIDS cases a causal relation between chlamydia infection and SIDS could not be demonstrated.