Introduction 3 5 7 16 3 5 6 12 10 11 14 1 4 We experienced unexpected effects of lenalidomide in del(5q) MDS patients that are suggestive of the mode of action in this patient subgroup and may have implications for future use of the substance in this patient population. To our knowledge, these types of responses to lenalidomide have not been previously described. Study design Between November 2003 and May 2006, 43 patients with del(5q) MDS with or without additional chromosomal abnormalities were treated with lenalidomide at St. Johannes Hospital, Medizinsche Klinik II, Duisburg, Germany. As of December 27, 2005, lenalidomide has been approved for clinical use in the United States, but not in Europe. These patients received lenalidomide as participants of clinical trials or through an expanded access program. Of the cases reported in this paper, only one patient took part in a lenalidomide clinical trial, and that patient was a non-responder. Patients were informed of inclusion of their information in the present report and consent was given. Results and discussion Patients with complex karyotypes Patient 1 9 2 17 1 Table 1 Sequential bone marrow cytogenetic analyses Patient number Analysis date Karyotype 1 10/2005 15 7 1/2006 8 7 3/2006 3 17 2 3/2004 1 12/2004 (FISH) No evidence of 5q31 deletion 6/2005 (FISH) No evidence of 5q31 deletion 1/2006 No evidence of 5q31 deletion 3 2/2004 46, XX, del(5)(q13q33) [20] 5/2005 12 13 4 5/2004 46, XY, del(5)(q13q33) [20] 12/2005 13 8 5 1/1996 15 10/2002 No change, non-mosaic pattern of del(5q) 3/2003  12/2003  6/2004  11/2004  06/2006  (FISH) indicates that additional fluorescence in situ hybridization was performed. Patient 2 6 1 Patients with transfusion independence despite long-term interruption of medication Patient 3 1 Patient 4 1 Patient with no response during treatment but transfusion independence after discontinuation Patient 5 trans 1 Discussion 10 11 8 6 10 Two other patients at our institution had interrupted lenalidomide treatment after prolonged intake (at months 24 and 13, respectively) in complete hematologic and cytogenetic remission for reasons other than an adverse event. Both patients remain in complete hematologic remission, ongoing for more than 6 and 21 months. Thus, altogether six out of 43 patients at our site have interrupted lenalidomide treatment for non-toxicity reasons. Five experienced an ongoing erythroid response and one patient relapsed after 3 months. 10 11 11 15 trans 8 13 In conclusion, although the above case reports obviously represent only selected cases out of a larger number of lenalidomide treated del(5q) MDS patients, these unusual and unexpected cytogenetic and erythroid responses suggest that some patients with complex karyotypes including del(5q) or who discontinue therapy may still benefit from lenalidomide treatment.