In 50 cases of sudden infant death cervical, paratracheal and lung hilar lymph nodes, the thymus and the spleen were investigated by histology and immunohistochemistry (CD 20, 21, 45RO). The cases were divided into 3 groups based on autopsy findings including extensive histology: A --without pathological changes (N = 12), B --with minimal to intermediate inflammation (N = 23) and C --with severe inflammation (N = 15). In accordance with previous results the frequency of "pathological" lymph node changes, such as paracortical lymphoid hyperplasia and variegated hyperplasia of the pulp increased from group A to group C. The B-cell antigens reacted accordingly. A pronounced lymphodepletation of the thymus as a sign of a long lasting stimulation of the T-cell system was also observed increasingly from group A to C. In summary, in none of the cases results obtained were indicative of a defect of the T- or B-cell system. The results in group A seem to indicate that changes in the reaction pattern of the lymphoid tissues could be a more sensitive method of detection of early stages of inflammation than local histology.