Clonal chromosome anomalies may be found in the bone marrow (BM) of patients with Shwachman syndrome, who are at risk to develop myelodysplastic syndromes and/or acute myeloid leukemias. In particular, an isochromosome i(7)(q10) is frequent, and is usually monitored by chromosome analyses. We tested an approach by real-time quantitative polymerase chain reaction (RQ-PCR) on a chromosome 7 polymorphism. Five DNA samples of 2 Shwachman syndrome patients with clonal i(7)(q10) in the BM were used. Both were heterozygous for the diallelic indel polymorphism MID1064, which maps in 7q35. The percentage of i(7)(q10)-positive cells was extrapolated from the ratio of the 2 alleles measured by means of an allele-specific RQ-PCR assay. The results were compared with cytogenetic analyses on the same material used for RQ-PCR. In 1 patient, the RQ-PCR results matched well with those of chromosome analyses, whereas in the other one RQ-PCR showed that around 40% of the BM cells were abnormal, while they resulted to be nearly 80% with conventional monitoring assays. As the results obtained by RQ-PCR refer to the DNA of around 128,000 BM cells, our method proved to be feasible and more efficient in the quantitative evaluation of the i(7)(q10)-positive clone than conventional ones.