Serial trephine biopsies were performed in 45 cases of chronic granulocytic leukaemia (CGL) in order to determine the frequency and significance of secondary myelofibrosis in the evolution of the disease. Histological changes were graded 1-5b, ranging from no increase in reticulin to dense osteomyelosclerosis. Many cases showed a progressive increase from Grade 1 to Grade 3, and accelerated disease, or blast crisis, often supervened when Grade 3 changes were present. However, a significant number of cases showed Grade 4 and 5 changes, which were indistinguishable histologically from those found in agnogenic myeloid metaplasia (AMM) (idiopathic myelofibrosis), at the time of diagnosis. These patients did not always show a rapidly fatal course and may be considered as an example of 'transitional myeloproliferative disorder', with features intermediate between CGL and AMM.