Sudden infant death syndrome (SIDS) is the most common cause of postneonatal mortality in the UK. Pathological investigations have shown evidence suggestive of respiratory obstruction with subsequent hypoxia leading to death. We examined 48 infants who died of SIDS and 30 who died of other, non-pulmonary, causes and identified pulmonary eosinophil and neutrophil leucocytes, mast cells, and T and B lymphocytes by immunocytochemistry. Positively stained cells were counted in the parenchyma and around the bronchi without knowledge of the tissue source. The results showed three times more eosinophils in the lungs of infants who died of SIDS (27.61 vs 7.91 [99% CI 1.76-5.87] cells/mm2 for parenchyma) accompanied by increased T lymphocytes and B lymphocytes. There were more peribronchial mast cells in the SIDS group (22.1 vs 14.7 [1.03-2.10] cells/mm2) and insignificant differences in neutrophils and parenchymal mast cells. There were significant associations between eosinophil, B lymphocyte, and T lymphocyte numbers. These findings provide evidence for an abnormal T lymphocyte-mediated pulmonary inflammatory response in SIDS. Products of eosinophil degranulation can cause epithelial damage and pulmonary oedema, which could cause the respiratory obstruction and hypoxia associated with SIDS.