Cytogenetic analysis has proven to be a mandatory part of the diagnosis of myelodysplastic syndromes (MDS) as well as a major indicator for predicting clinical course and outcome. Aside from the 5q-syndrome, no specific clinico-cytogenetic entity has been reported. To determine the incidence and clinical significance of acquired abnormalities of chromosome 17 in adult primary MDS, we reviewed the cytogenetic features of 271 patients detected at our institution during a 10-year period. Clonal cytogenetic abnormalities were identified in 109 cases. Among them, abnormalities of chromosome 17 were identified in 13 patients (11.9%). Five patients had "single" defects, while in eight patients abnormalities of chromosome 17 were associated with other chromosomal rearrangements ("complex" defects). After chromosomes 5, 7, 8 and 1, abnormalities of chromosome 17 were the most frequent chromosomal rearrangements in our patients with MDS. Following "single" defects of chromosome 17 were identified: del(17)(p12) in two cases, and i(17)(q10), del(17)(q21;q23) and del(17)(q12;q22) in one case each. Two patients with del(17p), one with RAEB-t and the other one with CMML, had an aggressive course of the disease with accelerated leukemic transformation and short survival. Patient with i(17q) had RARS subtype and died soon after diagnosis, while other two cases with interstitial deletions of the long arm of chromosome 17 had RAEB subtype and stable, no progressive course of the disease. Among "complex" karyotypes with abnormalities of chromosome 17 we identified der(17) in four, monosomy 17 in two, and del(17p) and l(17q) in one case each. Most of these patients transformed to acute leukemia and had very short survival. The results of this study suggest that abnormalities of chromosome 17 are frequent finding in MDS. Loss of genetic material in 17p, both in "single" and "complex" defects, seems to be closely related to poor prognosis of MDS patients.