Allogeneic haematopoietic stem cell transplantation in humans results in true biological chimeras. Whilst circulating haematopoietic cells become donor genotype after transplantation, other cells believed to remain recipient in origin. It was only recently realized that bone marrow-derived cells may also contribute to non-haematopoietic tissues, suggesting a level of plasticity not previously expected. New concepts in ontogenesis and developmental potential of adult haematopoietic stem cells emerged and called into question the long-standing biological dogma of lineage commitment. Great expectations for novel therapies were engendered both by the scientific and popular press. This review discusses the current status of the heated debate on the "plasticity" of adult stem cells.