Twenty-two sudden infant death syndrome (SIDS) cases and 22 controls were examined immunohistochemically with regard to IgA, IgM, and IgG plasma cells in tracheal wall and duodenal mucosa. Furthermore, the presence of secretory component in tracheal surface and gland epithelium as well as in duodenal crypt and villus epithelium were evaluated. The examined specimens were obtained at autopsies. The control groups consisted of 11 infants who died of noninfectious causes and 11 who died of infections. In the tracheal wall, the SIDS group had higher IgM cell numbers than the control group that died of noninfectious causes (p less than 0.01), whereas the SIDS victims had lower IgA and IgM cell numbers than the infectious control group (p less than 0.01). In the duodenal mucosa, the SIDS group had significantly higher IgA cell numbers than the noninfectious control group (p less than 0.02) but lower IgA cell numbers than the infection group (p less than 0.01). Secretory component was present in the epithelium from all SIDS cases and controls, both in the tracheal wall glands and in the duodenal crypt mucosa. These findings indicate that the mucosal immune system is stimulated in SIDS.