The haematological findings in the peripheral blood of 50 patients in whom the diagnosis of chronic granulocytic leukaemia (CGL) had been made in the haematology laboratory, and who were subsequently shown to be Philadelphia-chromosome-positive, have been reviewed. The differential leucocyte counts were performed by 3 observers, examining a total of 1,500 cells in each patient. The degree of anaemia at diagnosis was unrelated to sex and correlated poorly with leucocyte count; thrombocytopenia seemed unrelated to leucocytosis. A differential luecocyte count which included a complete spectrum of granulocytic cells, with prominent peaks in the percentages of myelocytes and neutrophils, was an invariable finding. Absolute basophilia occurred in all patients and absolute eosinophilia in 92%. In 54% of the patients there was an absolute lymphocytosis. Unlike the finding in normal subjects, there was no linear relationship between the numbers of circulating neutrophils and monocytes. Application of these findings should improve the accuracy of the haematological diagnosis of CGL, while study of the rare cases which possess the above features but are Ph1-negative may throw further light on the role of the Philadelphia chromosome in the natural history of CGL.