We used fluorescence in situ hybridization (FISH) to determine MYC and chromosome 8 copy number on whole nuclear imprint preparations of 24 breast carcinomas, seven benign breast samples, and two phyllodes tumors. None of the benign tissues and neither of the phyllodes tumors demonstrated an increased copy number for MYC or chromosome 8, which was defined as greater than two signals in > 10% of nuclei. In contrast, 22 of 24 carcinomas demonstrated an increased MYC copy number. The modal numbers of MYC copies/nucleus were 0-2 in seven cases (29%), 3-5 in seven cases (29%), 6-9 in five cases (21%), and > 9 in five cases (21%). An increased chromosome 8 copy number was observed in 21 of 22 carcinomas with MYC gain, and the modal number of signals/nucleus was either identical to (n = 14; 64%) or less than (n = 8; 36%) the number of MYC copies. The number of MYC copies correlated with cellular DNA content, as determined by using flow cytometry. In peridiploid tumors (DNA index 0.9-1.2; n = 7), the MYC copy numbers/nucleus were 0-2 in five cases and 3-5 in two cases. In contrast, the modal MYC copy numbers/nucleus among the 11 hyperdiploid tumors (DNA index 1.3-1.9) were 0-2 in one case, 3-5 in four cases, 6-9 in five cases, and > 9 in one case. All three tetraploid/hypertetraploid carcinomas exhibited > 9 MYC copies/nucleus. We conclude that an increased MYC copy number, as detected by using interphase cytogenetics, is extremely frequent in human breast carcinomas. However, in most cases, MYC gene duplication is probably secondary to polysomy of chromosome 8 and/or genomic endoreduplication (i.e., DNA aneuploidy).