Hematologic abnormalities were studied prospectively in 38 patients with brucellosis. Anemia was found in 74% of patients, leukopenia in 45%, neutropenia in 21%, lymphopenia in 63% and thrombocytopenia in 39.5%. Eight patients (21%) were pancytopenic|; seven of these individuals also had splenomegaly. Bone marrow hypoplasia was not found. Bleeding complications developed in 26% of patients and were significantly associated with clotting abnormalities (low platelet count, low fibrinogen level, and/or prolongation of thrombin clotting time); i.e., bleeding occurred in ~50% of patients with marked clotting abnormalities but in no patients with normal clotting. Determination of fibrinogen levels at different stages of brucellosis led to a redefinition of the normal level for patients with this infection. Patients without clotting abnormalities had fibrinogen levels of 233-711 mg/100 ml (mean, 384 mg/100 ml), whereas patients with thrombocytopenia and prolonged thrombin clotting time had levels of 122-360 mg/100 ml (mean, 216 mg/100 ml; P < .001) that increased to 233-519 mg/100 (mean, 360 mg/100 ml) when clotting values returned to normal. Lymphopenia was significantly correlated with the severity of clinical manifestations (bleeding and hepatic involvement).