Aneuploidy is an important prognostic factor in many cancers. Chromosome 1 abnormalities are present in most breast carcinomas. These may be part of a non-specific increase in DNA (aneuploid status) or represent a restricted chromosomal abnormality. In 16 breast carcinomas we compared chromosome 1 aneusomy with ploidy status. Patients were selected from a mammographically screened population and interphase tumour nuclei were studied by in situ hybridization using a chromosome 1 pericentromeric probe. Ploidy status was assessed by image cytometry on disaggregated cells from paraffin blocks. Of eight cases showing chromosome 1 aneusomy, six (75%) were aneuploid and two diploid. Six (75%) of the eight eusomic cases were aneuploid. This study demonstrates that chromosome 1 aneusomy does not always reflect a gross aneuploid status but, in some tumours, is part of a more restricted chromosomal abnormality. Interphase cytogenetics, possibly using a small panel of pericentromeric probes, may be more sensitive than DNA cytometry for detecting abnormal nuclear DNA content.