To investigate the relationship between common fragile sites and sister chromatid exchange (SCE), lymphocyte cultures were treated with aphidicolin and bromodeoxyuridine (BrdU) and analyzed using a sequential G--~SCE staining protocol. A total of 1 163 SCEs were mapped to their corresponding G-band sites, which were assigned to one of the following four categories: fragile sites expressed; fragile sites nonexpressed; nonfragile sites with breaks; or nonfragile sites with no breaks. The designated common fragile sites were found to be preferred locations for SCE formation, not only when these sites were "expressed" as visible gaps or breaks, but even when they were "nonexpressed" in the cell. SCEs were also more likely to occur at nonfragile sites with breaks than at nonfragile with no break sites. Further, SCEs were found to be distributed nonrandomly across fragile sites and nonfragile sites, and among the fragile sites, the high frequency SCE sites were highly correlated with the high frequency breakage sites. These data support the hypothesis of common steps in the mechanism of aphidicolin-induced SCE formation and common fragile site expression.