Cytogenetic analysis was performed on eight osteosarcomas, including six primary untreated biopsies, one second primary in a patient with a history of undifferentiated sarcoma, and one recurrent lung metastasis. Two primary tumors and the peripheral blood lymphocytes from all eight patients had normal karyotypes. Six of the tumors demonstrated extremely complex karyotypes, with modal numbers in the hypodiploid, triploid, and hypertetraploid ranges. The predominant types of structural abnormalities observed were nonreciprocal translocations and deletions, which differed between cases. A consistent loss of normal chromosome 13 homologs was evident in the six cases with abnormal tumor karyotypes; however, chromosomal loss was not restricted to #13. Molecular studies of osteosarcoma, especially with regard to the retinoblastoma locus on chromosome 13, should take into consideration the complex cytogenetic changes seen in this tumor.