The putative oncogenes BCL-1, BCL-2, and BCL-3 are commonly rearranged by translocations to the immunoglobulin genes in B-cell malignancies. However, Southern blotting rarely detected their involvement in chronic lymphocytic leukemia (CLL). This discrepancy could stem from some unique features of the oncogenesis of CLL or be due to shortcomings of Southern blotting. We have therefore evaluated the role of fluorescence in situ hybridization (FISH) in the detection of these oncogenes in CLL. Twenty consecutive CLL patients were studied by FISH for the detection of BCL-1, BCL-2, or BCL-3 rearrangement and for the presence of trisomy 12. Selected patients were also evaluated by classical cytogenetic techniques and by Southern blot analysis. Juxtaposition of JH and BCL-1 was demonstrated in 10 (50%), BCL-2 in three (15%), and BCL-3 in four (20%) of the patients. Trisomy 12 was detected by FISH in 11 (55%) patients. The coexistence of trisomy 12 and translocation of the BCL-1 oncogene was common. Three of the patients had chromosomal aberrations compatible with those detected by FISH. In contrast, in none of the five patients selected by their positive FISH findings was a rearrangement demonstrated by Southern blotting. We conclude that FISH is a sensitive method for the detection of oncogene involvement in CLL. Mainly BCL-1, but also BCL-2 and BCL-3, are commonly translocated to the immunoglobulin heavy chain locus on chromosome 14. These translocations are often associated with trisomy 12. These findings indicate that the BCL oncogenes are commonly involved in CLL and lend support to the multi-hit theory of cancer development.