Interphase cytogenetic analysis using chromosome-specific probes is increasingly being used to detect chromosomal aberrations on paraffin-embedded tissue sections. However, quantitative analysis of the hybridization signal is confounded by the nuclear slicing that occurs during sectioning. To determine the sensitivity and accuracy of chromosome in situ hybridization for detecting numerical chromosomal aberrations on paraffin-embedded sections, in situ hybridization was performed on sections derived from mixtures of cell populations with known frequencies of numerical chromosomal aberrations and the Chromosome Index (CI) was calculated (i.e., total number of signal spots/number of nuclei counted) as a quantitative measure of chromosome copy number. The presence of 25% or more monosomic or tetrasomic cells in a given population was easily detected as a change in CI (P < 0.05). Lower degrees of polysomy could be detected as a small percentage of nuclear fragments with > 2 signal spots. The CI was not significantly influenced by a change in section thickness from 4 to 8 microM, by an increase in cell size from 478 to 986 microM3, or by the choice of detection method (fluorescence vs. conventional bright-field microscopy). Comparative analysis of touch preparations and tissue sections from the corresponding breast tumors showed that CI accurately reflects the average copy number of chromosomes in intact nuclei and may actually be superior to in situ hybridization on whole nuclei for the detection of numerical chromosomal changes in defined histologic areas. This method is thus a sensitive and accurate means of studying genetic changes in premalignant and malignant tissue, and of assessing the genetic changes associated with specific phenotypes.