Chromosome counts were performed on 1,100 cells from 17 malignant breast carcinomas and on 168 cells of four normal tissue samples after amethopterin treatment and G-banding. Karyotypes were established from 216 cells of 11 tumor-derived cultures and from 47 cells of four nonmalignant tissue-derived cultures. Karyotypes of cells from nonmalignant samples showed a normal diploid chromosomal constitution with no consistent loss or gain of a specific chromosome. Structural chromosomal abnormalities were not observed. Tumor-derived cultures could be distinguished from normal cultures on the basis of a significantly increased incidence of numerical changes and structural chromosomal aberrations. In nine of 11 tumor-derived cultures, numerically normal cells were shown to be pseudodiploid, with frequencies ranging to 43% (mean, 13.2%) of the diploid cells. In agreement with previous reports, cytogenetic analyses showed predominantly diploid cells. Clonal numerical changes of chromosomes 17, 18, 20, and 21 could be detected in three tumor samples. Clonal structural abnormalities could be observed in two of 11 analyzed tumours. A t(6;12)(p21;p13) and an enlarged chromosome 7 (7q+) were found in a patient with invasive ductal carcinoma. An inversion of chromosome 7 [inv(7)(q11.2q32)] was observed in one case, also diagnosed as invasive ductal carcinoma. The significance of these findings in relation to clinical data is discussed.