In many patients with myelodysplastic syndromes or acute myeloid leukemia, complex chromosome aberrations can be seen, among which aberrations of chromosome 5 constitute a substantial part. With conventional cytogenetic technique, these aberrations are often identified as deletions or monosomy 5. We analyzed nine patients who, under conventional cytogenetic analysis, showed deletion or monosomy 5. We used fluorescence in situ hybridization with whole-chromosome painting probes to identify the counterpart chromosome and locus-specific identifiers for 5q31 and 5q33 approximately q34. A deletion of 5q was found concomitant with unbalanced translocations. Our results and cases from the literature showed that material from chromosome 5 could be translocated to almost all chromosomes. All patients but one had short survival; this one patient had a preserved 5q31 and 5q33 approximately q34 but a deletion of the q-arm more centromeric than these bands. In eight of the nine patients, further 14 translocations were revealed, not involving chromosome 5.