Our nuclear pharmacy has reported that a failed radiochemical purity (RCP) (i.e., RCP < 90%) of 99mTc-sestamibi may be associated with the use of a first elution at later stages from a long-ingrowth time (i.e., > or = 72 h) wet-column generator. The primary purpose of this study was to evaluate the effects of 99mTc eluates from wet- and dry-column generators on the RCP of 99mTc-sestamibi under the above conditions. RCP values were found to be measurably higher and kit failure rates lower with the use of dry-column generator eluate. Using a dry-column generator eluate, Cardiolite kits were prepared with 11.10 GBq of 99mTc at 3, 4, and 5 h postelution and 5.55 GBq at 6, 10, 11, and 12 h postelution. Our data suggest that when 11.10 GBq of 99mTc from a dry-column generator with > or = 72-h ingrowth was used to prepare 99mTc-sestamibi, kit failure started to occur using 99mTc eluate at approximately 4 h postelution. When 5.55 GBq was used to reconstitute the kit, RCP failure began to occur using 99mTc eluate approximately 10 h postelution and wet-column generators; the failure rate can be reduced even further by avoiding the addition of high activities of 99mTc and long elution times.