In 1987, a 50-year-old patient presented with isolated thrombocytopenia (27,000/mm3) which proved to be refractory to steroid medication and high i.v. doses of immunoglobulin. Two years later he developed macrocytic anemia. Chromosomal analysis confirmed the diagnosis of myelodysplastic syndrome (MDS), refractory anemia type with blast excess. Cytogenetically, three cellular populations were observed: one normal (75% of metaphases) and two abnormal, clone A (2%) 46,XY, del(5q), del(11q), and clone B (23%) 46,XY, del(5q), del(11q) plus 2 other anomalies. Evolution was characterized by worsening of the bicytopenia with marked hypoplasia of the megakaryocytic and erythroid series while the percentage of blasts remained stable. Concerning the chromosomal markers, the normal population disappeared and clone A became predominant (clone A 97%, clone B 3%). This case shows that isolated thrombocytopenia can be the sole initial manifestation of MDS. We discuss the possibility that "refractory thrombocytopenia" constitutes a diagnostic category like refractory anemia or refractory anemia with ring sideroblasts. The proliferative advantage of clone A or the disadvantage of clone B may be due to the occurrence of new, cytogenetically non-detectable mutations.