A retrospective review was conducted of yellow fever vaccination among laboratory workers receiving annual serologic assessment to determine the initial and long-term response after boosting. Patients were divided into three groups based on pre-vaccination serology: Group 1,1: 10: Group 2, 1:20-1:40 and Group 3, > 1:40. The percent with >= four-fold increase in titers after booster vaccination were: 78% (646/829, Group 1), 65% (79/12 1, Group 2) and 10% (8/79, Group 3) (p < 0.0001). The median times to titer failure (< 1:40) were 798 days (Group 1), 3340 days (Group 2) and 7709 days (Group 3) (p < 0.0001). Pre-vaccination serology influenced the initial and long-term response to yellow fever booster vaccination. Published by Elsevier Ltd.