Background. We described a patient with chronic lymphocytic leukemia (CLL) and lung cancer and unusual chromosomal aberrations. Case report. At the same time with the diagnosis of B-cell CLL, squamocellular lung carcinoma diagnosis was established. Using interphase fluoresecence in situ hybridization technique (FISH) we detected monosomy 12 and deletion of 13q34 occured in the same clone. One month after the beginning of examination the patient died unexpectedly during sleep immediately before we applied a specific treatment for CLL or lung carcinoma. Conclusion. Simultaneous occurrence of monosomy 12 and deletion of 13q34 in a patient with B-cell CLL has been described only once before, but as a part of a complex karyotype. The prognostic significance of these abnormalities remains uncertain.