A method for the quantification of nuclear DNA in thick tissue blocks by confocal scanning laser microscopy is presented. Tissues were stained en bloc for DNA by chromomycin A3. Three-dimensional images, 60 Fm deep, were obtained by stacking up confocal fluorescent images obtained with an MRC-BOO (Bio-Rad, Richmond, CA). The effects due to bleaching and attenuation by depth of fluorescence emission were corrected mathematically. The DNA contents were estimated by summing up the detected emission intensities (discretized into pixel gray levels) from each segmented nucleus. Applications to an adult rat liver and to a human in situ carcinoma of theesophagus are shown to demonstrate, respectively, the precision of the method and its potential usefulness in histopathology. Comparisons are made with DNA histograms obtained on the same materials by image cytometry on smears and by flow cytometry. Ploidy peaks obtained with the confocal method, although wider than with other methods, are well separated. Confocal image cytometry offers the invaluable advantage of preserving the tissue architecture and therefore allowing, for instance, the selection of histological regions and the evaluation of the degree of heterogeneity of a tumor.Key terms: DNA cytometry, confocal laser microscopy, histology, 3-D There is a considerable interest in DNA cytometry, notably because of the importance of searching for correlations of nuclear DNA content with cancer prognosis in individual patients (e.g., 22, 40, 62).The two methods commonly used a t present for nuclear DNA content analysis, image cytometry (ICM) and flow cytometry (FCM), are not devoid of limitations. Observing the tissue architecture may be important in identifying regions for cytometry and, therefore, the main shortcoming of ICM on smears or imprints and of FCM is obviously that they do not allow the study of DNA contents in situ. Even ICM on histological sections does not allow this, as the tissue structure is observed in projection and the resulting nuclear profiles are usually either incomplete if the section is thin or overlapping if the section is thick enough to include a sufficient proportion of complete nuclei. Finally, preparation procedures for ICM and FCM may produce a selection bias of a given cell type (9) and/or nuclear debris (27).'This paper is an expanded version of a work presented at the