In an attempt to contribute to the knowledge of the natural history of Philadelphia-chromosome-positive chronic myeloid leukaemia (CML) and its prognosis, we analyzed sequentially the myeloid differentiation antigens of peripheral blood neutrophil granulocytes (NG) in different evolutive stages of the disease. Four monoclonal antibodies (CD15, CD24, 31D8, and 13F6) were used, and a total number of 116 sequential studies were performed in 43 patients. At diagnosis, there is a significant decrease of NG expressing myeloid differentiation antigens, which recover to nearly normal levels after initial control of the disease. The onset reduction is probably due to the circulation of incompletely mature NG. In accelerated/blastic phase NG expressing myeloid differentiation antigens decrease again, probably due to a true antigen loss. This reduction could herald by a few months the development of accelerated/blastic phase. In such a case, its predictive strength is higher than that of the well recognized initial prognostic parameters in CML. These results indicate that the sequential study of NG myeloid differentiation antigens may contribute to both a better understanding of the natural history of CML and the evolutive prognosis of this disease.