Chromosomal abnormalities are detected by conventional cytogenetic or FISH analysis in 50% of chronic lymphocytic leukaemias (CLL). Trisomy 12 and del 13q14 account for 70% of these abnormalities. The incidence of these two abnormalities was studied in CLL patients by Southern blot analysis using a highly purified B-cell malignant population (CD5 > 95%, CD3 < 5%). Probes for the D13S25 marker on chromosome 13 band q14 and for the RBTN3 gene on chromosome 12 band p12-13, were used. Deletion of the D13S25 was detected in 17/42 patients (43%) in a homozygous (9.5%) or heterozygous (30%) configuration. Deletion of the D13S25 marker appears to be a clonal and early event in CLL development since it is detected in > 95% of the malignant clonal population. Conversely, trisomy 12 is rarely a clonal event (5/33 patients, 15%) and a varying proportion of cells carrying this abnormality can be demonstrated in 30% of CLL patients (10/33 patients).