The recently described monocytoid B-cell lymphoma is a low-grade lymphoma presenting most frequently in elderly women and commonly associated with autoimmune diseases. Leukaemic expression of this disease has been reported in advanced stages. A case of monocytoid lymphocytosis without lymph node enlargement is presented herein. A 60-year old woman complaining of easy bruises was found to have a 2-cm splenomegaly. Her laboratory data included the following: haemoglobin, 125 g/L; haematocrit, 0.35 L/L; white cell count, 29 x 10(9)/L with 32% PMN, 3% stabs, 2% myelocytes, 1% metamyelocytes, 30% lymphocytes and 32% atypical mononucleated cells showing wide, pale cytoplasm neatly contoured and oval nucleus with monocytoid features. The basal coagulation study showed prothrombin 50%, APTT 40 seconds, fibrinogen 68 mg/dL and FDP between 80 and 160 ng/dL. Splenomegaly without lymph-node enlargement was found on CT scan. The bone-marrow biopsy showed a 68% monocytoid lymphocytic infiltration, acid-phosphatase positive and tartrate-sensitive, without fibrosis. Bone-marrow and peripheral immunophenotype showed those cells to be CD22, CD 19 and CD11 positive, while T and CD25 markers were absent. The patient was treated with alpha-2b interferon at a dose of 3MU three times a week for 6 months, with general improvement and regression of the leukaemic expression. Eleven months after diagnosis she died of a central nervous system haemorrhage. The morphological, immunological and cytochemical features of the monocytoid lymphocytes in this case are commented, along with their variable behaviour. A review of the literature is also carried out, attention being laid on the onset and the response to therapy of B-cell monocytoid lymphomas as the singularity of this case lies on its exclusively leukaemic onset. It is concluded that an interrelationship between monocytoid B-lymphocytic leukaemia and B-cell monocytoid lymphoma might possibly exist, such as that between chronic lymphocytic leukaemia and diffuse lymphocytic lymphoma.