Cytogenetic analyses of hematological neoplasms have proved to be relevant in the diagnosis, treatment, and prognosis of afflicted patients. Breast cancer (BC) cytogenetics is expected to contribute in the same fashion. Literature has shown that clone-specific chromosomal changes do occur in BC, yet, their clinical significance is unknown. Most of the studies have been conducted on patients with advanced stage tumor. Karyotypic analyses of the few reported cases from stage I BC tumors revealed a higher frequency of single clonal abnormalities. This work describes an ongoing BC cytogenic study on samples from stage I tumors to enhance and clarify this observation. Included as control are chromosomal analyses of peripheral blood and peritumoral normal tissue samples of these patients, which might provide information regarding predisposing cytogenetic aberrations. Non-random chromosomal abnormalities in BC include involvement of chromosomes 1, 3, 6, 11, 16, and 17. Three groups of non-random chromosomal alterations, ranging from specific abnormalities, partial monosomies, and secondary changes (eg, numerical loss of chromosomes) are described. Survival appears to be more favorable in patients without complex karyotypes. A better understanding of the clinical and etiologic implications of BC is expected to emerge from continued assessment of breast tumor cytogenetics.