A comparison of tumor ploidy by flow cytometry (FL) and cytogenetics (CYG) was made in 86 fresh pediatric solid (n = 47) and hematopoietic (n = 39) tumors using simple sampling and semiautomated proprietary FL preparation methods and defined histogram interpretive criteria. Tumor karyotypes with 44-48 chromosomes were regarded as CYG diploid and other chromosomal complements CYG aneuploid for comparison purposes. Five histograms were uninterpretable and nine cases failed to produce 15 or more metaphases for karyotyping. Mean G0/G1 peak coefficients of variation of all 86 cases were 2.7 and 3.0 for the diploid and aneuploid populations, respectively. Of the 72 eligible cases, 41 were concordant diploid and 16 concordant aneuploid with an overall concordance of 79%. The DNA index and karyotypic index correlation coefficient was 0.92 for the 16 concordant aneuploid cases. Analysis of the 15 discordant cases highlights the limitations of both methods and of the histogram interpretive criteria and indicates that FL is probably more sensitive for detection of tumor aneuploidy as defined and detected by these methods.