To analyse the cytological features of benign monocytoid B-cells, we investigated, histologically and immunohistochemically, 78 reactive lymphadenopathies expressing monocytoid B-cell reactions. Within a broad cytological spectrum of reactive monocytoid B-cells, two cytological types can be recognized: 1 the common type composed of medium-sized cells with irregular or bean-shaped nuclei and inconspicuous nucleoli, and 2 the large cell type composed of cells considered to represent large transformed monocytoid B-cells, with less pleomorphic round nuclei with vesicular chromatin and moderately basophilic, prominent nucleoli. In between these variants transitional forms showing evolution to large transformed monocytoid B-cells occurred. While monocytoid B-cell reactions in 70.5% of all cases were composed predominantly of the common type of monocytoid B-cells, in 29.5% of cases the large transformed cells prevailed. The two distinctive cytological types of reactive monocytoid B-cells seem to have their neoplastic counterparts in monocytoid B-cell lymphoma, including its large cell variant.