Fifty patients, eventually diagnosed as having brucellosis, were studied prospectively for comparison of a simple culture of bone marrow aspirate from the iliac crest (0.5–1.0 ml) with two cultures of blood (5–10 ml) taken 30–60 min apart and with a tube-agglutination test. Cultures of bone marrow and blood were positive in 92% and 70 070 of the patients, respectively (P <.001). Bacteria multiplied significantly faster in bone marrow cultures (4.32 vs. 6.65 days; P <001). All isolates were identified as Brucella melitensis biotype 1. Serological diagnosis was established in 86% of the patients. The efficacy of cultures of blood decreased significantly with chronic and subacute forms of infection, whereas that of bone marrow culture decreased only in chronic forms. Prior use of antibiotics reduced the positivity of cultures of blood but did not affect bone marrow culture. Bone marrow culture is recommended for patients with fever of unknown origin, negative serology, and unexplained articular or hematologic involvement, and patients in whom brucellosis is suspected. © 1986 Oxford University Press.