Campylobacter fetus subspecies jejuni (CBJ) has been recently recognized as a common pathogen in bacterial gastroenteritis in children. During a period of 16 months, 51 cases of C fetus subspecies jejuni gastroenteritis were diagnosed. Five of the children in whom the cases were diagnosed were previously known to be immunodeficient: two had X-linked agammaglobulinemia, one had agammaglobulinemia, one had combined immunodeficiency, and one had transient hypogammaglobulinemia. Average duration of fever and diarrhea was longer in the five immunodeficient children (15 and 23 days, respectively) compared with the normal children (four and five days, respectively). Excretion of C fetus subspecies jejuni in stool persisted for 20 to 27 days in four of the immunodeficient children and for one year in the fifth, whereas normal children excreted C fetus subspecies jejuni for only four to 16 days. Campylobacter fetus subspecies jejuni may be added to the list of bacterial pathogens most likely to infect immunodeficient children, especially those with a defect of the humoral system.