Myelodysplastic syndromes (MDS) are a group of disorders characterized by dyshematopolesis in bone marrow (BM) and peripheral blood (PB) cytopenias. In recent years particular attention has been paid to myeloproliferative disorders with dysplastic features or myelodysplastic syndromes that evolve into a myeloproliferative disorder. The present study was designed to analyze patients with MDS but with a normal or increased colony forming capacity, in order to see whether or not cell cultures could contribute to the diagnosis of intermediate MDS-MPD conditions. A total of 80 patients diagnosed as having MDS were included in the study. CFU-GM assay was performed by plating 1 x 10 (5) mononuclear cells/mL in IMDM and 0.9% methyl-cellulose containing 10% PHA-LCM. In all cases cultures were run in parallel without PHA-LCM to assess autonomous growth. Cultures were incubated at 37 degrees C in a fully humidified atmosphere with 5% CO2 and scored at day 14. Cytogenetic analysis was performed according to standard procedures. Short-term cultures of 24 and/or 48 hours were used. Twenty-two patients out of the 80 MDS cases included in the study showed a normal or increased cell growth pattern. Among these 22 patients, eight were diagnosed as suffering from chronic myelomonocytic leukemia (CMML) according to the FAB criteria and were excluded from the present analysis. The remaining 14 cases, which constitute the body of this study, displayed an increased number of clusters and/or colonies, with an altered cluster/colony ratio (anomalous growth) in 10 cases. Autonomous colony formation was present in five of these 14 cases and autonomous cluster growth was seen in all but three of them. In addition, one patient showed endogenous BFU-E growth. Morphological diagnoses were then revised due to this aberrant colony growth pattern: based on actual criteria, 3 patients could have been considered as having a-CML (atypical chronic myeloid leukemia). Another 6 cases evolved to a more proliferative disorder: 5 to CMML, and one to a-CML. Interestingly, in 3 of these 6 patients the evolution took place concomitantly with an infectious episode. In one additional patient the platelet count increased up to 1000 x 10 (9)/L and required treatment with hydroxyurea. Our results show that intermediate MDS-MPD cases are relatively common and that in vitro characteristics, ie high clonogenic capacity with a high cluster/colony ratio and scanty autonomous growth, in patients showing myelodysplastic features could contribute to an early diagnosis in these cases. It is possible that in some cases an infectious episode, through higher cytokine secretion, contributes to the development of these disorders.