We aimed to identify and classify cases of paediatric myelodysplastic syndromes (MDS) occurring in Britain to estimate the incidence of this rare group of diseases, investigate the results of therapy and identify prognostic risk factors. Patients aged below 15 years at diagnosis were collected from England, Scotland and Wales, inclusively between 1990 and 1999. One hundred and thirty-five patients were accepted as de novo MDS or juvenile myelomonocytic leukaemia (JMML). The incidence for this period was 1.35 per million (age standardized rate) which is below that reported outside the UK. The overall survival was 45%[standard error (SE) = 4%] at 5 years: 40% (SE = 6%) for JMML and 50% (SE = 6%) for other MDS. Significant adverse prognostic factors for JMML were a platelet count < 40 x 10(9)/l, raised fetal haemoglobin, FPC score and age above 2 years at diagnosis, for other MDS only monosomy 7 was significant. To conclude, the incidence of MDS/JMML in children in the UK appears to be lower than that reported outside the UK. This may be either a real difference in incidence or variation in reporting. Monosomy 7 is associated with poor outcome in MDS other than JMML. The prognosis of JMML depends on age, platelet count and fetal haemoglobin.