Fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH) allow cytogenetic analyses of primary tumors without culture. CGH allows detection and mapping of allelic imbalance by simultaneous in situ hybridization of differentially labeled tumor (green fluorescing) and normal DNA (red fluorescing) to a normal human metaphase spread. Regions of increased or decreased copy number in the tumor are mapped onto the normal metaphase chromosomes as increases or decreases in the green to red fluorescence ratio. This technique gives a comprehensive assessment of gene dosage imbalance throughout the tumor. However, it is limited, at present, to fairly large tumors containing few normal cells. FISH, on the other hand, allows analysis of DNA sequence copy number at specific loci in single nuclei. A wide variety of DNA probes is available for FISH, including chromosome-specific probes which hybridize to alpha-satellite pericentromeric DNA regions (to detect changes in specific chromosome copy number and overall ploidy) and specific locus probes targeting 20-150 kilobase sequences (to detect specific amplifications, deletions, breakpoints, or rearrangements). FISH using these probes has been applied to interphase nuclei in touch preparations, smears from fine needle aspirates, and thin (< 6 microns) and thick (> 20 microns) sections cut from formalin-fixed, paraffin-embedded tissue. Analysis of thick sections allows accurate actual signal enumeration within the histological context. This approach may allow analysis of subtle premalignant, early malignant, and infiltrating tumors in which malignant cells must be differentiated from nonmalignant cells.(ABSTRACT TRUNCATED AT 250 WORDS)