Sixteen patients with chronic granulocytic leukaemia (CGL) in blast cell transformation (BT) were studied by means of serial bone marrow trephine biopsies (BMB). The BMB were performed during the following stages of the disease: 1 at diagnosis of BT, 2 following ablative therapy and 3 during haematological recovery following autografting. In 10 of the 16 patients, BT was recognized by the presence of a distinctive infiltration with blast cells with a single large hyperchromatic nucleolus. In four of the 16 patients, the diagnosis of BT could only be made by BMB since simultaneous marrow aspiration yielded either a dry tap or no marrow fragments; in two of these four patients the BT was focal in the BMB specimen. Following ablative therapy and during haematological regeneration after autografting, BM aspirates were unsatisfactory in most cases and they were inadequate to assess cellularity or the presence of residual blasts. In contrast, sections of BMB showed clearly whether a decrease in cellularity took place or, in some, residual blasts were still present. BMB samples obtained 2 weeks after autografting showed haemopoietic regeneration consisting of discrete foci of either erythroid, granulocytic or megakaryocytic precursor cells. We conclude that BMB is essential for diagnosing CGL in BT, for monitoring the progress of these patients after therapy and in assessing the haemopoietic regeneration following autografting.