AIMS--To investigate the value of bone marrow biopsy in the diagnosis of mycobacterial infection. METHODS--The culture results of 433 bone marrow samples taken between 1983 and 1992 were reviewed. The histopathology reports on bone marrow trephine specimens of culture positive samples and all those on HIV positive patients sent in 1992 were also reviewed. RESULTS--Fifty one specimens yielded Mycobacterium spp, 47 were obtained from HIV positive patients. Of the isolates, 42 were Mycobacterium avium-intracellulare (MAI), five were M tuberculosis (MTB), and the remaining four comprised a variety of atypical mycobacteria. All MAI positive samples were obtained from HIV positive patients, with the bone marrow being the only culture positive specimen in one third. Bone marrow yielded MTB only in patients from whom it was also isolated in other specimens. Eleven of 47 trephine specimens from positive bone marrow showed granulomata and nine showed acid-fast bacilli. No acid-fast bacilli were seen in the absence of granulomata. CONCLUSION--Bone marrow biopsy for mycobacterial culture should be reserved for severely immunosuppressed patients and should not be advocated for immunocompetent patients with suspected tuberculosis. Bone marrow biopsy still has a role in the investigation of pyrexia of unknown origin in HIV positive patients, despite the advent of mycobacterial blood culture techniques, particularly if these can be processed safely in automated systems.