Thirteen patients with simple monosomy 7 presented with pallor in 11, easy bruisability in five, splenomegaly in four, no infections, refractory anaemia in all, granulocytopenia in seven, monocytosis in three, leucocytosis in four and thrombocytopenia in eight. Peripheral blood and bone marrow findings were consistent with myelodysplastic syndrome (MDS) type I in three, type II in two, type III in two, type IV in three and acute myelofibrosis in three patients. Transformations to acute leukaemia in seven patients were M2 in one, M4 in four, megakaryoblastic in one and undifferentiated in one. Lack of chromosome 7 in 12-85% of analysed cells at initial presentation of MDS progressed to nearly 100% during blastic transformation. At that time an additional change in the long arm of chromosome 3 was seen in two patients and trisomy 8 in 6% of analysed cells in a third case. The median survival time was 12 months for MDS and 3 months for acute leukaemia. Simple monosomy 7 appears to be largely confined to young children and elderly people.