The Danish Thorotrast Study was recently reestablished and improved. The cohort has been reidentified and followed up, and now comprises 1003 Thorotrast-exposed patients. For all suspected haematological cases, cytological and histological material has been revised and malignant diseases have been reclassified. The numbers of cases of leukemia and other related haematological disorders were as follows: 16 acute myeloid leukemia (AML); 8 myelodysplasia syndrome (MDS); 1 acute lymphatic leukemia (ALL); 3 chronic myeloid leukemia (CML); 4 non-Hodgkin's lymphoma (NHL); 2 multiple myeloma (MM); 2 myelofibrosis (MF); 2 chronic lymphatic leukemia (CLL). Except for CLL, all cases might be Thorotrast-induced. (Expected number of leukemias: <2.5.) The findings in the German, Japanese, Portuguese and Danish studies are very similar. Some of the characteristic features include a high incidence of AML with several erythroleukemias, many cases of MDS, and a relatively low incidence of CML. In several studies of leukemia induced by alkylating agents, erythroleukemia is also described as a prominent feature. The possibility exists that a phase of relative predominance of erythroid elements in the bone marrow may be a common and not an unusual feature in the pathogenesis of these secondary leukemias. The findings are also compared with histopathological data from a Danish control group of de novo leukemia patients and from atomic bomb survivors with radiation-induced leukemia. The relative frequency of AML is higher among the Thorotrast-exposed patients than among the Danish control group and the A-bomb survivors. In contrast, low relative frequencies are seen for ALL and CML in Thorotrast cases in comparison with de novo leukemia cases and A-bomb survivors. It can be concluded that differences in relative and absolute frequency of leukemias and myelodysplastic syndrome exist not only between the irradiated populations and the unexposed control group, but even between groups exposed to low-LET (linear energy transfer) and high-LET radiation.