Interphase cytogenetics by means of in situ hybridization with the chromosome 12-specific biotinylated alpha satellite DNA probe pSP 12–1 was used for the study of trisomy 12, the most common chromosomal abnormality in chronic lymphocytic leukemia. In situ hybridization was performed on methanol/acetic acid fixed cells of conventional cytogenetic preparations from eight patients and on morphologically and immunologically classified cells of cytospin preparations from seven patients. The results show that trisomy 12 is more common than assumed on the basis of karyotype analysis of metaphase chromosomes: 2 of 13 patients with a normal karyotype in G-banding analysis were shown to have trisomy 12 by interphase cytogenetics. Immunophenotyping of the cells of one patient showed that the trisomy was restricted to cells with Ig light chain clonality. For the evaluation of the prognostic, therapeutic, and biologic significance of trisomy 12, in situ hybridization should be used in parallel with karyotype analysis because it allows the study of all cell populations of both interphase and mitotic cells, whether neoplastic or normal.