The in situ hybridization technique (ISH), using specific chromosome DNA probes on stained bone marrow or peripheral blood smears, allow us to correlate morphological findings with the cytogenetic abnormalities detected by conventional cytogenetic analysis. We have applied this methodology to study three patients, two of them diagnosed of acute lymphoblastic leukaemia (ALL) and one with a myeloid blast crisis of chronic myeloid leukaemia (BC-CML). Conventional cytogenetic analysis showed, among other chromosomal abnormalities, a trisomy 8 in two cases and a trisomy 7 in one case. ISH demonstrated that cytogenetic clonality was restricted to the lymphoblasts in both cases with ALL. In the BC-CML patient, myeloid blasts as well as mature and semimature granulocytic elements showed three hybridization signals. ISH correlating morphological and cytogenetic findings may be a useful technique for lineage as well as for the follow-up of the haematological patients.