A recent immunohistochemical study found increased numbers of eosinophils, but no mast cells, in the pulmonary parenchyma of infants who died of sudden infant death syndrome (SIDS). The present study tested the hypothesis that this pulmonary eosinophilia could be IgE-mediated. Histomorphometry was used to compare the numbers of eosinophils, mast cells, and IgG-, IgA-, IgM- and IgE-expressing lymphoid cells in the lungs of two groups of infants. Twenty-eight subjects aged less than 1 year were selected from post-mortem records of infant deaths between 1989 and 1992. Fourteen were cases of SIDS and these infants were matched for age and gender to 14 controls who died of other non-pulmonary conditions. Immunohistochemical stains were used and positive cells were counted on six peribronchial and six subpleural fields. The numbers of eosinophils in both peribronchial and subpleural regions were significantly higher in SIDS compared with controls (P = 0.0071 and P = 0.041, respectively). The numbers of IgA-expressing lymphoid cells were also significantly increased in SIDS cases (P = 0.042). There were no differences in IgG, IgM or IgE expression or in mast cell numbers. These results confirmed that pulmonary eosinophils are increased in SIDS, but not through an IgE-mediated pathway.