Necrotizing lymphadenitis with Warthin-Finkeldey type giant cells was found in the inguinal lymph nodes of a 14-month-old boy following measles vaccination. Electron microscopy displayed (mostly in the endothelial cells) numerous intracytoplasmic inclusions composed of fine, dense granular and fibrillar material, which (among other structures) are known to occur within various cells in connection with measles paramyxovirus infection. Subsequent investigation disclosed familial immunodeficiency of a predominantly cellular type.