Defined by isolated del 5q and no excess of marrow blasts, the '5q- syndrome' is a specific type of myelodysplastic syndrome (MDS) with particular characteristics, including severe anaemia, frequent thrombocytosis, typical dysmegakaryopoiesis and favourable outcome. Its pathogenesis remains uncertain, particularly regarding the role of the inactivation of gene(s) situated in 5q. Until the advent of lenalidomide, repeated red blood cell (RBC) transfusions were generally the only treatment for 5q- syndrome, which was resistant to other therapeutic approaches. Lenalidomide can lead to RBC transfusion independence in at least two-thirds of cases of 5q- syndrome, and about one half of those responses are maintained after 2 years of treatment. Importantly, frequent complete pathological and cytogenetic responses are also obtained. Grade 3 or 4 neutropenia and thrombocytopenia, especially during the first 6-8 weeks of treatment, are the major side effects of lenalidomide, justifying close clinical and blood counts monitoring. Preliminary results suggest that lenalidomide is also active in MDS with del 5q other than the 5q- syndrome. The mechanisms of action of lenalidomide, although uncertain, appear to include targeting of the del 5q clone. Therefore, lenalidomide may have an effect on disease course and survival, which is currently being assessed in clinical trials.