Familial myelodysplastic syndrome (MDS) has been claimed to account for as many as one third of children with MDS, especially among those showing monosomy 7. The present study is the first to provide population-based estimates of the risk of haematological and other malignancies in relatives of children with MDS. The study was extended to include children with acute myeloid leukaemia (AML) and chronic myeloid leukaemia (CML). The index group consisted of 46 children with MDS, 62 with AML, and eight with CML, which is thought to represent all myeloid leukaemias in Danish children, 1980-91. By linkage to the Central Population Register we identified parents (230), siblings (231), grandparents (151), aunts and uncles (132) and cousins (140). Information on the cancer incidence was obtained from the Danish Cancer Registry. 27 cancers were observed versus 26.7 expected (relative risk 1.0). Leukaemia in relatives was observed in only one family. None of 11 children with MDS and monosomy 7 had family members affected by leukaemia. We found no evidence of an increased overall risk of cancer in the relatives. The risk of familial MDS may be considerably lower than previously estimated.