Monosomy 7 is frequent in acute myeloid leukaemia (AML) and in preleukaemic dysmyelopoietic syndromes but often it is not the only chromosome anomaly associated with these conditions. We report 14 patients with "pure" monosomy 7 and their clinical and haematological data are analysed in order to clarify the possible implications of this chromosome anomaly. The following points are considered: 1) In spite of the apparent variability of clinical forms in which monosomy 7 is found, several characteristics are common to all monosomy 7 patients, i.e. the presence of a preleukaemic phase and blood and marrow features suggesting the early involvement in the disease of all marrow cell lines. The different diagnoses associated with monosomy 7 are correlated with different steps of a unique myeloproliferative disease whose typical course can be reconstructed. 2) Monosomy 7 has a negative prognostic value. When it is found in a preleukaemic disorder it indicates a high risk of progression to AML, while in AML it implies recurrent infections, poor response to therapy and short survival. 3) The significance of the lack of Colton blood group antigens in monosomy 7 patients is discussed, with particular regard to the fact that the patients in whom this lack was found are the only ones who had not received transfusions in the months before the tests were done. 4) The finding of defective neutrophil chemotaxis in monosomy 7 patients is confirmed and the clinical importance of this fact is emphasized. 5) The data on the 14 patients support the opinion that AML, in general, is heterogeneous in origin. It is postulated that monosomy 7 is a marker of a specific pathogenetic pathway of AML, which implies the beginning of the malignancy in a pluripotent stem cell.