A previously healthy 10-year-old Greek boy born to nonconsanguineous healthy parents developed progressive liver disease after acute infectious mononucleosis. EBV-induced autoimmune hepatitis was suspected and treatment was started with high-dose prednisolone, acyclovir and intravenous immunoglobulins. Despite therapy his liver function continuously deteriorated and the child died 9 months later in profound immune deficiency from candida septicemia. Flow cytometric analysis of his lymphocytes revealed a major subpopulation of atypical cells (20.3%) which were CD3+, fitted into the lymphocyte gate bur showed a very low level of CD4 expression, comparable to that of monocytes. After short-time cell culture, the cells became adherent and developed granules and dendrites; We conclude that these cells may represent strongly activated CD4+ T lymphocytes with downregulated CD4 expression or a subtype of dendritic cells.