Transitional B cells have been recently identified in human peripheral blood. However, their precise role in human B cell differentiation has not been established. Therefore, besides characterizing them further in the blood of healthy adults and children and cord blood, we used the immune reconstitution after hematopoietic stem cell transplantation (HSCT) model to define their role in human B cell development. Human transitional B cells are reliably identified as CD19(+) CD24(high) CD38(high) lymphocytes and represent approximately 4% of B cells in healthy adult peripheral blood. In contrast, they are abundant in cord blood (near 50% of B cells) and their percentage progressively decreases during infancy. Similarly, after HSCT, all B cells first appearing in peripheral blood are transitional B cells; afterwards, the transitional B cell percentage progressively decreases while the mature naïve B cell proportion rises. Our results now formally demonstrate that transitional B cells are necessary developmental intermediates for human mature B cell generation.