Chronic myelomonocytic leukaemia (CMML), a disorder belonging to the group of myelodysplastic syndromes, has a number of peculiar features which raise the question as to whether it should be considered a distinct entity in its own right. The problems associated with its classification and diagnosis are discussed in this report using all currently available tools from clinical data to molecular genetics, including morphology, histology, cellular biology and cytogenetics. Three groups of patients can be identified (isolated monocytosis with a mild degree of dysplasia, severe cytopenia and the most frequent type with proliferative symptoms dominating the clinical picture). The latter group is close to atypical chronic myeloid leukaemia and perhaps these two entities should be regarded as a single one. Classification of the disease is further complicated by the possibility of evolution from one subgroup into another one and by the finding that CMML can also arise as a disorder secondary to other myeloproliferative (MPS) or myelodysplastic (MDS) syndromes. No specific marker of the disease has been identified by cytogenetics or molecular biology. Due to all these facts, we believe that CMML should perhaps be viewed more pragmatically by considering the use of prognostic factors that could at least help to define different groups of patients who may require different therapeutic strategies. We conclude that CMML is a heterogeneous syndrome with features of both MPS and MDS, encompassing primary and secondary stem cell disorders and varying widely in its clinical presentation. This heterogeneity should stimulate the search for reliable predictors of evolution which would allow a better definition of CMML subtypes based on prognostic factors.