In group B (n=79), the 5q deletion had been observed by cytogenetics and was confirmed to be present in all cases by fluorescence in situ hybridization of 5q31.CONCLUSIONS: Fluorescence in situ hybridization of 5q31 detected the 5q deletion in 6% of cases without clear evidence of del(5q) by conventional banding cytogenetics. We suggest that fluorescence in situ hybridization of 5q31 should be performed in cases of a suspected '5q- syndrome' and/or if the cytogenetic study shows no metaphases or an aberrant karyotype with chromosome 5 involved (no 5q- chromosome).