Monoclonal B-cell lymphocytosis (MBL) is a precursor of chronic lymphocytic leukemia (CLL). Observations of MBL in blood donors raise concern that transmitted MBL may cause recipient CLL. Using a database with health information on 1.5 million donors and 2.1 million recipients, we compared CLL occurrence among 7413 recipients of blood from 796 donors diagnosed with CLL after donation cessation, and among 80, 431 recipients of blood from 7477 matched CLL-free donors. During follow-up, 12 and 107 cases of CLL occurred among the exposed and unexposed recipients, respectively, yielding a relative risk of 0.94 (95% confidence interval, 0.52-1.71). Analyses using the entire database showed no evidence of CLL clustering among recipients of blood from individual donors. In conclusion, when donor MBL was approximated by subsequent donor CLL diagnosis, data from 2 countries' entire computerized transfusion experience over more than 30 years indicate that MBL/CLL transmission does not contribute importantly to recipient CLL risk.