The syndrome defined as ‘idiopathic CD4 lymphocytopenia’ (ICL) is a rare disease of unknown aetiology, often associated with severe depression of immune defences and the occurrence of opportunistic infections. A case is reported wherein a severe immunodeficiency syndrome with persistent idiopathic CD4+ lymphopenia developed in a woman suffering from systemic microscopic polyarteritis; no signs of HIV 1/2 or HTLV I/II infection were evident. The patient died of widespread opportunistic infections. The association of ICL with vasculitis has never been reported until now. A link between the two diseases cannot be ruled out.