The effects of etoposide (VP-16) on sister chromatid exchange (SCE) and chromosome abnormalities were studied by using human peripheral lymphocytes. This drug produced a significant increase in the SCE frequency and chromosomal aberrations such as breaks, exchanges, and tetraploidy with or without endoreduplication. Analysis of the breakpoints of chromatid deletion with respect to their association with SCE in the metaphases from the second division demonstrated that VP-16-induced deletions arose with little dependence on the site of SCE. It is suggested that chromatid deletions induced by this drug derive from unrepaired chromatid breaks rather than from incomplete exchanges.