Purpose Complex karyotype (CK) is the poorest risk factor in patients with myelodysplastic syndrome (MDS). It has recently been reported that monosomal karyotype (MK) worsens the prognosis of patients with CK. Patients and Methods We analyzed 1,054 adult patients with MDS with an abnormal karyotype from the Spanish Registry of MDS. The aim of the study was to describe the incidence, characteristics, and prognosis of MK; the main end points were overall survival (OS) and leukemia-free survival. Results MK was identified in 172 patients (16%), most of whom (88%) presented with CK. Variables significantly associated with OS were age (hazard ratio [HR], 1.90; P = .001), bone marrow (BM) blast percentage (HR, 1.05; P < .001), hemoglobin level (HR, 1.71; P < .001), platelet count (HR, 1.41; P < .001), karyotype complexity (CK [three abnormalities]: HR, 1.81; P = .003; very CK [> three abnormalities]: HR, 2; P < .001), and abnormalities of chromosome 5 and/or 7 (HR, 1.89; P < .001). Variables significantly related to the risk of transformation to acute myeloid leukemia (AML) were higher BM blast percentage (HR, 1.12; P < .001) and karyotype complexity (CK: HR, 2.53; P = .002; very CK: HR, 2.77; P < .001). Conclusion After accounting for karyotype complexity, MK was not associated with OS or evolution to AML. In conclusion, these results demonstrate that the prognostic value of MK in MDS is not independent and is mainly the result of its strong association with number of chromosomal abnormalities. © 2013 by American Society of Clinical Oncology.