In this paper we have analysed the behaviour of myelodysplastic marrow in a long-term bone marrow liquid culture system (LTBMC) from eleven patients with myelodysplastic syndromes with regard to cellularity, day-7 and day-14 CFU-GM growth, cluster formation, adherent cells and CFU-F formation. An altered CFU-GM pattern was found in 64% of cases at diagnosis, while normal growth was seen in the remaining cases, all of which were affected by refractory anaemia. The levels of CFU-GM, as well as cellularity, were reduced in myelodysplastic marrows compared to normal controls over the whole duration of LTBMCs. Cases with a normal CFU-GM level at diagnosis also showed pathological behaviour when examined in LTBMC. The duration of dysplastic haematopoiesis was significantly shorter than that of controls. The proliferative ability of CFU-F was reduced in 50% of cases as shown by replating experiments. In conclusion, myelodysplastic marrow shows an abnormal behaviour in LTBMC, even in those cases which present normal CFU-GM growth at diagnosis.