Trephine biopsies of 101 chronic myelocytic leukaemia (CML) patients were analysed to study the relationship between initial and subsequent histological features vis-a-vis clinical behaviour of the disease. The patients with blast crisis at presentation were excluded. At diagnosis 62 (61.4%) patients revealed granulocytic-megakaryocytic (gran-meg) proliferation whereas granulocytic (gran) proliferation was found in 39 (38.6%) patients. Gran pattern at diagnosis was associated with shorter survival and early evolution into blast crisis (36.8%) in 12 months, although the difference in the total incidence of blast crisis between the two histological groups was not statistically significant. Myelofibrosis was detected in more number of cases on follow up (89.1%) as compared to the initial biopsies (80.2%). However myelofibrosis did not correlate with initial cellular composition, overall survival or the phase of CML (P > 0.05). Transition from one histological type to another was observed in 15 out of 60 (25%) cases while remaining in the chronic phase.