We, and others, previously reported the expansion of CD5+CD19+ B-cells after allogeneic hematopoietic stem cell transplantation (HSCT). Recently the equivalent of the B1 cells in mice have been described in human as CD20+CD27+CD43+CD70- B cells. Here we report that while 20% of CD5+CD19+ cells in controls were CD43+, over 75% of CD5+CD19+ were CD43 positive in patients whether or not they previously developed chronic GvHD (p=0.0001). CD5+CD19+ B cells, CD5+CD43+CD19+ B cells and B1 cells counts were significantly decreased in patients who previously developed chronic GvHD and this effect of GvHD affected CD5+ and CD5- similarly within the B1 cell subset. Thus our results strongly suggest that the previously reported expansion of the CD5+CD19+ population might be related to an expansion of the B1 cell subset, and that B1 cell reconstitution was impaired in patients who developed chronic GvHD.