Tubular carcinoma of the breast is a well-differentiated variant of invasive ductal carcinoma and has been shown to have an exceptionally favorable prognosis, as manifested by a low incidence of lymph node metastases and an excellent overall survival. It is unknown whether this subtype represents an early step along the continuum of development to a more aggressive, poorly differentiated ductal cancer, or whether these cancers are destined to remain well differentiated with limited metastatic potential. We undertook an analysis of 18 pure tubular carcinomas of the breast using comparative genomic hybridization to evaluate the chromosomal changes in these tumors. An average of 3.6 chromosomal alterations of the genome were identified per case. The most frequent change involved loss of 16q (in 78% of tumors) and gain of 1q (in 50% of tumors). All but one case with 1q gain also exhibited a concomitant 16q loss. Other frequent changes involved 16p gain in 7 of 18 cases (39%) and distal 8p loss in 5 of 18 cases (28%). Comparison with known genomic alterations in a mixed group of invasive cancers shows tubular cancer to have fewer overall chromosomal changes per tumor (P < .01), higher frequency of 16q loss (P < .001), and lower frequency of 17p loss (P = .007). These results strongly suggest that tubular carcinomas are a genetically distinct group of breast cancers. © 2001 by W.B. Saunders Company.