Clinicobiological, histological, cytogenetic and molecular genetic studies were performed in a case of atypical B-cell chronic lymphocytic leukaemia (B-CLL) with the t(11;14)(q13;q32) evolving into Richter's syndrome (RS) in order (a) to determine the clonal relationship between the cell of origin for B-CLL and RS, and (b) to analyse genetic events underlying the disease progression in this patient. After 4 years following diagnosis, a rapid deterioration of the clinical picture occurred, concomitant with the appearance of large lymphoid blasts in peripheral blood (PB), bone marrow (BM) and ascites samples. A diagnosis of RS was made and cytogenetic analysis revealed karyotype evolution with trisomy 7 and del(17p) in addition to t(11;14). Fluorescence in situ hybridization showed 78% lymphoid blast cells obtained from ascites sample to be trisomic using a chromosome-7-specific pericentromeric probe. Whereas no rearrangement of the c-myc proto-oncogene was detected at disease progression, direct sequencing of p53 gene exon 5-9 revealed an exon 7 missense point mutation. This abnormality was not present in the CLL phase. Immunological staining with the monoclonal antibody PAb-1801, detecting the p53 protein product, revealed a negative pattern in the CLL phase, whereas 24% positivity was documented in representative samples obtained at RS. It is concluded that RS was cytogenetically related with B-CLL in this patient, suggesting the occurrence of a bona fide transformation and that the mutation of p53 exon 7, in association with the development of 17p deletion, possibly played a role in the development of RS.