Body iron disorders have been reported after myeloablative conditioning in patients undergoing hematopoietic stem cell transplantation (HSCT). There is a concern that labile plasma iron (LPI), the redox-active form of iron, can be involved in the occurrence of toxicity and other complications commonly observed in the early post-HSCT period. In order to better understand the LPI kinetics and its determinants and implications, we undertook sequential LPI determinations before and after conditioning until engraftment in 25 auto-HSCT patients. Increased LPI was present in only 5 patients before starting conditioning. Shortly after conditioning, LPI levels were increased in 23 patients, with peak at day 0, returning to normal range upon engraftment in 21 patients. Overall, LPI levels correlated weakly with serum ferritin and more strongly with transferrin saturation; however, both parameters were apparently not applicable as surrogate markers for increased LPI. Although this was a small cohort, logistic regression suggested that baseline LPI levels could predict occurrence of grade III or IV toxicity. In conclusion, LPI kinetics is influenced by aplasia following conditioning and engraftment. Measuring LPI before starting conditioning can offer an opportunity to predict toxicity and, perhaps, the need for chelation therapy.