We studied the association between myelodysplastic syndromes (MDS) and malignancies in a cohort of 155 patients with MDS, 21 of whom presented malignant solid tumors. Myelodysplasia was present after the diagnosis of cancer in eight patients (interval between the diagnosis of both conditions 18 months, median survival 49.5 months), simultaneously with diagnosis in 11 (median survival 8 months), and before malignancy in two patients (interval between the diagnosis of both conditions 47 and 7 months). One patient was given chemotherapy for lung cancer, and three patients received radiotherapy for adenocarcinoma of the kidney and cancer of the prostate. At the time of diagnosis of MDS, nine patients already presented metastatic spread. Fourteen patients died, ten as a result of tumor-related complications and four because of transformation to acute nonlymphocytic leukemia. The analysis of the incidence of malignancy in patients with MDS was statistically significant for males, and the relative risk was significant in both sexes. The results of this study show that MDS patients present a higher incidence of malignant tumors than the general population, that MDS may be of real paraneoplastic significance, and that the occurrence of MDS in cancer patients may be considered to be related to the malignancy rather than an independent phenomenon.