Bone marrow trephine biopsy specimens from 24 patients with chronic granulocytic leukaemia were studied to identify bone marrow changes that suggest incipient or impending blast transformation. In the chronic phase of chronic granulocytic leukaemia, blasts and promyelocytes were seen predominantly in the paratrabecular and perivascular regions, while myelocytes, metamyelocytes and segmented polymorphs were seen predominantly in the central intertrabecular marrow space. The patients were divided into two equal groups according to the degree of perivascular and paratrabecular infiltration: those with minimal (one to three layers of blasts and promyelocytes) and those with marked (four to eight layers of blasts and promyelocytes) infiltration. Ten of the 12 patients in the former group did not develop blast transformation and remained in the chronic phase during a 32- to 84-week follow-up period, whereas nine of the 12 patients in the latter group developed blast transformation, with six undergoing transformation within the first 26 weeks. These findings suggest that the histomorphological pattern of bone marrow changes in chronic granulocytic leukaemia may be useful in recognizing a subset of chronic granulocytic leukaemia patients undergoing blast transformation in the ensuing 6 months.