Globin chain synthesis was studied in the reticulocytes of 30 patients with various myelodysplastic syndromes (MDS) to determine the ca:f globin chain synthetic ratio and its probable prognostic value.The mean (SD) value of the total az:# ratio was 0l82 (0 45) ranging from 0 05 to 1-73. The same ratio in 10 normal controls was 1 01 (0 04). This difference was significant. Furthermore, the %:P ratios were lower than normal in 14 patients (acthalassaemia-like) (group I), almost within normal limits in 11 (group II), and higher than normal in five (fithalassaemia-like) (group III). In each group almost all the FAB subtypes were represented. The addition of exogenous haem in several of the test samples resulted in a slight to pronounced increase in the m:f ratios, particularly in group I. In 92% of the high risk cases (refractory anaemia with excess blasts (RAEB), chronic myelomonocytic leukaemia (CMML)) or 87-5% of patients who finally developed acute non-lyphoid leukaemia (ANLL) low or normal a:f ratios were found. No significant correlation was noticed between a:f ratios and various haematological variables or survival.It is concluded that in MDS the z:f ratio varied enormously across the entire population of patients, as well as within each FAB subtype, thereby restricting its prognostic value. Although haem deficiency may be implicated in some cases of MDS, why this should be remains unclear.