Cytological and histological features of the bone marrow in Philadelphia-positive chronic myelogenous leukaemia (CML) at diagnosis were analysed in a prospective study. Formerly described (Frisch et al, 1984) subtypes GRAN and GRAN/MEG were found, and myelofibrosis and incipient blast crisis could also be detected. A more or less continuous spectrum of number and degree of atypias in megakaryocytes was found. Biopsy and imprint were complementary in making differential diagnosis of both subtypes. Cytologically, type GRAN/MEG was characterized by a mixed proliferation of neutrophil, eosinophil and basophil series, besides proliferation of megakaryocytes. Type GRAN showed a predominantly neutrophilic proliferation. The importance of performing bone marrow analysis in evaluation of CML patients is discussed.