Chromosomal studies were performed on 114 blood samples and 117 bone marrow samples, taken systematically over a period of 4 months after bone marrow transplantation (BMT) in 42 children grafted for acute lymphoblastic leukaemia (ALL) (n = 20), acute myeloid leukaemia (AML) (n = 16), non-Hodgkin's lymphoma (NHL) (n = 2) and myelodysplastic syndrome (MDS) (n = 4). In some cases, follow-up investigations were performed. In the first 4 months following BMT, mixed chimerism was frequently observed in blood of AML (25%), ALL (30%), NHL (100%) cases and in bone marrow samples of ALL (35%). The presence and relative number of a patient's own metaphases found shortly after transplantation was not related to leukaemia relapse and probably represents residual non-malignant haematopoietic precursor cells of the host. In only one child grafted for MDS with 45,XY, -7 karyotype was the marker clone still detectable in bone marrow at day +437 post-BMT. This patient shows no recurrence of the MDS and has a sustained haematological recovery at the time of writing, i.e. 4.5 years post-BMT. In 11 other patients, various structural chromosomal abnormalities (not related to the original leukaemia) were found in both peripheral blood and bone marrow. In three different patients structural anomalies were also found in bone marrow and blood samples from donor-derived cells. This indicates that, besides irradiation, there are other as yet unidentified factors (e.g. drugs), which are capable of inducing chromosomal anomalies in the post-BMT period.