Seventy-six patients with chronic myeloid leukaemia (CML) could be subdivided by core biopsy into chronic granulocytic leukaemia (CGL, n = 24) and chronic megakaryocytic granulocytic myelosis (CMGM, n = 52). By pure clinical definition 59 patients were grouped as classical CML and 17 showed a course which we termed atypical myelosis. The most reliable criteria for distinguishing between the classical and atypical forms were ALP-Index, peripheral leukocyte and platelet count and the estimated number of megakaryocytes in the bone marrow. The classical myeloses consisted of 40 per cent CGL and 60 per cent CMGM whereas the atypical consisted of CMGM only including all stages of fibrosis. Fibrosis was at the time of bone marrow biopsy found in 20 per cent of classical CML and in about 50 per cent of atypical myeloses. In classical CML Philadelphia chromosome could be detected in all the patients with CGL and in 50 per cent of those with myelofibrosis. Atypical myeloses did not exhibit Philadelphia chromosome. In 70 per cent of the cases it was possible to distinguish between CGL and CMGM by peripheral blood findings and bone marrow cytology.