In a group of 75 untreated patients with a typical B cell chronic lymphocytic leukaemia (B-CLL) (CD19+, CD5/CD19+, CD23/CD19+), the frequency and clinical significance of TP53 gene deletion and chromosome 12 trisomy were assessed. The studies of peripheral blood lymphocytes were conducted using interphase in situ hybridization technique. Clonality was identified when TP53 deletion or chromosome 12 trisomy was found in at least 10% of cells. From all 75 examined patients 32 individuals without any of the genetic aberrations were analyzed (Group I) and 30 subjects with TP53 deletion (Group II) were chosen. In the other 13 patients, discussed in the next paper, either chromosome 12 trisomy (Group III--seven subjects) or both chromosome 12 trisomy and TP53 deletion (Group IV--six subjects) were found. In the Group I, there has been no further contact with three patients, while in the Group II--with two individuals. In the Group I, one patient of 29 in the study (3%) died after 84 months (seven years) from the diagnosis, whereas in the Group II--nine subjects of 28 in the study (32%) died within 1-36 months from the diagnosis. In three of those patients in the terminal condition, cytogenetic studies were repeated revealing an increase of approximately 5% in the percentage of peripheral blood cells with TP53 deletion. The frequent presence of TP53 deletion detected in 48% of patients is surprising. It is generally thought that the aberration is found in 10-15% of clinical cases. The studies should be confirmed on a larger group of patients.