Chronic lymphocytic leukaemia is a disorder with variable clinical presentation, but with constant and easily recognizable morphological features. The majority of cases are of B-cell lineage, the cells being characterized by IgM and IgD surface markers. In occasional cases monoclonal gammopathies with idiotypes identical to those of the SIg are produced. Some cases of CLL may be confused with Waldenström's macroglobulinaemia. The clinical and morphological features of certain other lymphoproliferative disorders have been summarized. Finally, a progression of B-cell differentiation along an immunoexpressive spectrum has been described and the relative position of CLL defined. A correlation of cell morphology and function is possible in most cases. The possibility that CLL may terminate in an acute blastic crisis or histiocytic lymphoma is raised. It is emphasized that LPD include a wide range of diseases, some being non-malignant, others of questionable malignancy, and still others being frankly malignant. Clinical, morphological and immunological data are required for diagnosis in some cases.