The proceedings of the third workshop of the molecular biology/bone marrow transplantation (BMT) study group held in January 1991 in Verona, Italy. This workshop was convened to review progress in the application of molecular techniques to the diagnosis and follow-up of patients with chronic myeloid leukaemia (CML) as well as other haematologic malignancies. The results of polymerase chainreaction studies in 157 CML patients 1-90 months post BMT suggest that leukaemia is frequently detectable for the first 12 months but rarely detected thereafter except in patients known to have a high risk of relapse. In the acute leukaemias and lymphomas there is a rapidly increasing number of leukaemia-specific as well as clone-specific molecular markers now available for the detection of minimal disease. It may be possible to coordinate multi-center prospective studies to investigate the role of these markers in the diagnosis and follow-up of haematologic malignancies.