In 153 consecutive patients with myelodysplastic syndrome (MDS) the prognostic value of FAB-classification, cytogenetics, Bournemouth score, a history of previous radio- or chemotherapy and in vitro bone marrow growth were retrospectively analysed, for both acute nonlymphocytic leukaemia (ANLL) development and survival. Thirty-eight of the 153 patients (25%) showed progression to ANLL, 63 (41%) died during the myelodysplastic phase due to infection or bleeding and three (2%) received allogeneic bone marrow transplantation (BMT). Univariate analysis showed that the FAB-classification, in vitro growth pattern and differentiation, and cytogenetics had a predictive value for ANLL development and survival. The Bournemouth score was predictive only for survival. Most predictive for the development of ANLL were in vitro growth pattern and maturation. Patients with normal in vitro growth progressed to ANLL in 6% of the cases, in patients with hypoplastic or leukaemic growth 32.5% developed ANLL (P less than 0.0001). The ANLL incidence in patients with normally differentiated in vitro colonies was 14.5%, compared with a 52% incidence in cases showing no in vitro cell maturation (P = 0.001). The combination of growth pattern and differentiation revealed an ANLL incidence of 4.2% in cases of normal growth and differentiation, and 60.4% if the in vitro growth and/or differentiation was abnormal (P = 0.006). In vitro maturation was the only parameter predictive for ANLL development in multivariate analysis. From our data it is concluded that the predictive value of in vitro bone marrow culturing in patients with MDS can be increased by including in vitro maturation as a distinct parameter. The in vitro prognostic data can be important in selecting MDS patients for intensive chemotherapy or BMT.