Mycobacterial histiocytosis is a rare disease usually associated with haematological or immunological disorders. We report a fatal case caused by M. fortuitum infection showing the typical disseminated histiocytosis. Immunological investigations revealed impaired cellular immunity demonstrated by negative skin tests with different "recall-antigens", and in vitro an isolated defect of helper T-lymphocytes in the peripheral blood which in combination with hypergammaglobulinemia suggests a "lymphocyte and distribution syndrome".