A numerical alteration in chromosome complement in human dermal fibroblast cultures, hyperdiploidy with a normal occurrence of tetraploidy (IVH), has been reported to be associated with some hereditary single tumors including squamous carcinoma of the nasopharynx (NPC). Its incidence was compared in cultures derived from 39 NPC patients and 29 clinically normal subjects without a family cancer history by two different methods (percentage of numerically altered metaphases in chromosome preparations from nonconfluent monolayer Petri dish cultures in logarithmic growth, and by the distribution of DNA content of propidium iodide stained cells from plastic flask cultures in stationary growth phase as assayed by flow cytometry) to ascertain its usefulness in identification of such genetic predisposition. Concordance was observed between the two assays. There was a linear relationship between the percentage of hyperdiploid metaphases assayed in the chromosome preparations and the percentage of cells with a DNA index of greater than 1 as determined by flow cytometry. By metaphase assay, none of the 29 normals showed IVH. OF the 39 carcinoma of the nasopharynx patients studied, 19 had IVH and 20 did not. By flow cytometry there were significant differences in the flow cytometry DNA index (p less than 0.001) of IVH-negative (all normals and 20 carcinoma of the nasopharynx patients) and IVH-positive carcinoma of the nasopharynx patients. The percentage of cells with a DNA index of 2 and greater than 1 could be used to distinguish all IVH-negative from the IVH-positive subjects and, thus, were considered to be the parameters of choice in assaying IVH by flow cytometry. None of the subjects studied showed increased in vitro tetraploidy (IVT), which has been associated with some heritable colon cancer syndromes. Irrespective of family cancer history, approximately one-half of the NPC patients (19 of 39) had IVH, which has been reported to be associated with the in vivo expression of certain heritable tumors including carcinoma of the nasopharynx. The average age of carcinoma of the nasopharynx diagnosis was earlier (mean 52 yr) for the IVH-positive group than for the IVH-negative carcinoma of the nasopharynx group (mean 67 yr).