A bone marrow investigation is a common examination in HIV infected patients for the study of cytopenia, febrile syndromes of unknown origin and extension of neoplastic disorders. A study was made of bone marrow specimens from 35 patients with advanced HIV infection (stage IC or C, CDC, Atlanta) for morphologic and culture investigations (aerobes, anaerobes, fungi, and mycobacteria). In nine patients cytopenia accounted for the investigation of bone marrow specimens (9 aspirates and 3 biopsies); in only two cases did the investigation orientate towards a possible etiology: in the first patient a parvovirus B19 infection and in the second patient a hemophagocytic syndrome. In twenty-five patients the bone marrow specimen was studied because of fever of unknown origin (23 aspirates and 10 biopsies) and only in one case was the identification of Mycobacterium tuberculosis obtained. The other patient was studied for lymphoma staging and aspirate and biopsy examinations were normal. A high percentage of patients had eosinophilia, plasmacytosis, increased iron reserves, fibrosis, and changes consistent with myelodysplasia. In conclusion, in our experience the investigation of bone marrow specimen was of little help to clarify the possible etiology of cytopenia and febrile syndromes of unknown origin in patients with advanced HIV infection.