Bizarre parosteal osteochondromatous proliferation (BPOP), or Nora's lesion, is a rare tumorous lesion with aggressive growth that affects primarily the small tubular bones in the distal extremities and often recurs after excision. No previous cytogenetic data on BPOP are available. In the present study, lesions from 5 patients were investigated by chromosome banding and fluorescence in situ hybridization (FISH) analyses. Patient age ranged from 24 to 46 years, and the lesions were located in the fingers in 4 cases and in a toe in 1 case. Histological sections from all 5 tumors were characterized by a mixture of hypercellular cartilage, cancellous bone, and spindle cell components. Samples from 2 patients were available for cytogenetic analysis. One of these showed a normal female karyotype, and the other revealed a balanced translocation, t(1;17)(q32;q21), as the sole anomaly. The translocation was further characterized by 3-color metaphase FISH analyses, using 17 1q32-specific and 18 17q21-specific bacterial artificial chromosome probes, to map the precise location of the breakpoints. Split signals were detected by the RP11-99A19 probe in chromosome 1 and by the RP11-219F9 probe in chromosome 17. To determine whether these rearrangements are characteristic features of BPOP, paraffin-embedded tissue sections from all 5 patients were investigated by interphase FISH analyses. All 5 cases had a break in 1q32, and 4 of the 5 cases showed a break in the 17q21 region. The results strongly indicate that t(1;17)(q32;q21), or variant translocations involving 1q32, are recurrent and unique aberrations in BPOP. Several genes are located within the 2 sequences spanning the breakpoints, and further studies should be performed to determine whether any of these are involved in the formation of a fusion gene.