Non‐radioactive in situ hybridization (NISII) with a chromosome 12‐specific alpha satellite probe was performed on 20 patients with chronic lymphocytic leukaemia (CLL) with normal karyotype (15 cases) or with inadequate mitotic yield (5 cases) from mitogen‐stimulated cultures.

All patients had over 70% lymphocytes coexpressing the CD5/CD23 antigens. While less than 1% interphase nuclei showed three fluorescent spots in 16/20 patients, evidence of trisomy 12 in 15–25% interphase cells was detected in four patients. According to the FAB classification the diagnosis in these patients was typical B‐CLL, stage III (Rai's staging system) in one case, CLL/PLL, stage II and III in two cases, PLL, stage III in one case. In order to confirm these results, NTSH was repeated after 1 month in one patient and after 2 years in three patients. All patients had been treated with chemotherapy in the period between the two NISH experiments. In all cases a 1.8–3‐fold increase of percentage of trisomic interphase cells was detected.

These findings suggest that in B‐CLL clones with trisomy 12 may have proliferative advantage over clonal B‐lymphocyte without +12 and possibly, that they may be more resistant to chemotherapy.

We conclude that NISH is a sensitive technique allowing for the detection and monitoring of trisomy 12 in a fraction of B‐CLL patients with normal karyotype or with no analysable mitoses despite employment of polyclonal B‐cell mitogens.